Oso and Polar — Because Authorization Is Still Application Logic

Odo and Polar (Generated by ChatGPT) / Author

There is a particular stage in every software project where it is announced that we just need to check whether the user is allowed to do this.

And, then six months later the question has become: The user can do this if they’re an administrator of the organization, unless the resource belongs to another tenant, except if they’re a member of the owning team, unless the document is private, unless the request came from the API, in which case…

At which point somebody discovers that the original if user.is_admin has evolved into an unauthorized distributed policy engine.

This is roughly the problem Oso is trying to solve.

Oso is an authorization framework and platform, while Polar is its declarative policy language. Polar provides the language for expressing authorization logic; Oso provides the machinery for evaluating that logic and integrating it with applications.

And, unlike some authorization systems that begin by defining a standardized authorization model, Oso starts from a rather practical premise: Your application already has a data model. Your authorization rules need to operate on it.

That turns out to be a significant design choice.

Polar Is the Interesting Part

The easiest way to understand Oso is to start with Polar.

Polar is a declarative, logic-oriented language designed specifically for authorization. It has roots in logic programming and supports things such as rules, variables, pattern matching, relationships, and facts.

A very simple policy might look conceptually like this:

allow(user: User, "read", organization: Organization) if
    has_role(user, "member", organization);

This isn’t a configuration file describing a fixed RBAC model.

It is a rule.

And, that distinction becomes important very quickly.

You can define relationships between application objects:

"read" if "member";
"write" if "admin";

Or, express relationships between resources:

"read" if "owner" on "project";

The policy describes the logic, while the application supplies the objects and relationships against which that logic is evaluated.

That makes Polar feel considerably closer to a programming language than to the XML-heavy policy languages that preceded it.

Which, depending upon your perspective, is either a feature or the beginning of another problem.

The Oso Model

At the center of an Oso authorization decision is essentially the familiar question:

Can this actor perform this action on this resource?

Conceptually:

But, Oso deliberately doesn’t stop at those three objects.

Polar can reason about the relationships and attributes surrounding them.

That means the authorization decision can involve:

• Users
• Groups
• Organizations
• Tenants
• Roles
• Resources
• Parent resources
• Relationships
• Resource attributes
• Request context
• Application-specific facts

This is where Oso starts to look less like a conventional RBAC library and more like a general authorization programming model.

RBAC Without Making Everything RBAC

Role-based access control is easy to describe:

Real applications tend to produce something more like:

Now, authorization depends upon those relationships.

Oso treats this as normal.

For example:

resource Organization {
    roles = ["member", "admin"];
    permissions = ["read", "write"];

    "read" if "member";
    "write" if "admin";
}

The policy describes what the roles mean.

The application or authorization data describes who has those roles.

That separation is important.

Role assignment is data. Role behavior is policy.

This is a distinction that becomes increasingly useful as an application grows.

A role such as admin may exist in hundreds or millions of individual assignments, while the definition of what an administrator can actually do might be expressed by a relatively small amount of policy.

ReBAC Is Not an Afterthought

This is also where Oso overlaps with the Zanzibar/OpenFGA/SpiceDB world.

Consider:

A user might be allowed to read a repository because they are a member of a team that belongs to the organization that owns the repository.

That’s relationship-based authorization.

Oso can express this sort of relationship logic directly in Polar.

For example:

"read" if "member" on "organization";

The important thing isn’t the particular syntax.

The important part is that authorization can be expressed in terms of relationships between application objects, rather than requiring every relationship to be flattened into a gigantic permission table.

Oso’s current examples explicitly demonstrate combinations of RBAC, ReBAC, and ABAC rather than treating these as mutually exclusive authorization models.

That makes Oso particularly relevant to the sort of B2B application where everything is somebody’s resource inside somebody else’s organization.

So, most SaaS applications.

And, Then There Is ABAC

Roles aren’t enough for everything.

Suppose, a document can be read when:

user is an editor
OR
document is public
OR
user owns the containing project

That isn’t purely RBAC.

The document’s state matters.

Polar can express rules involving application attributes and facts.

Conceptually:

"read" if "editor";
"read" if is_public(resource);
"read" if "owner" on "project";

Now, the authorization decision is a combination of:

That’s ABAC, ReBAC, and RBAC all sitting in the same policy.

And, this is one of Oso’s more important characteristics.

It doesn’t require you to choose one authorization religion.

Facts

One of the more interesting concepts in Polar is the fact.

A fact is information available to the policy evaluator.

For example:

has_role(User:alice, "admin", Organization:acme)

or:

has_relation(Document:123, "project", Project:456)

The policy provides the rules.

The facts provide the state.

This produces an architecture that looks something like:

This is conceptually similar to the separation between policy and attributes in XACML, but the programming model is quite different.

XACML traditionally revolves around an attribute-based request and policy evaluation model.

Polar revolves around logic over application objects and relationships.

The Application Is Still Part of the Model

This is perhaps the biggest philosophical difference between Oso and something like OPA.

OPA deliberately tries not to care what your application considers a principal, resource, action, tenant, repository, invoice, Kubernetes object, or coffee machine.

OPA gives you a general policy engine.

Oso is much more interested in your application.

Your application has objects.

Your objects have relationships.

Your objects have attributes.

Polar should be able to reason about them.

Oso’s original library was explicitly designed to operate over application data, and Oso’s current local-authorization architecture continues that idea by allowing authorization decisions to combine centralized policy/data with data that remains in the application.

That is a very different design philosophy from: Here is some JSON. Good luck.

Policy Versus Application Code

One of the major reasons to introduce a policy language is to stop authorization logic from becoming scattered throughout application code.

Without a policy layer, you eventually get:

if user.is_admin:
    ...

followed by:

if user.is_admin or user.id == document.owner_id:
    ...

followed by:

if (
    user.is_admin
    or user.id == document.owner_id
    or (
        user.organization_id == document.organization_id
        and document.public
    )
):
    ...

And eventually:

if (
    ...
):
    # Nobody knows why this works anymore.

The policy approach is different:

allow(user, "read", document) if
    user_is_owner(user, document);
allow(user, "read", document) if
    document_is_public(document);
allow(user, "read", document) if
    is_org_member(user, document.organization);

Now, the authorization rules have somewhere to live.

They can be reviewed as authorization rules rather than discovered accidentally while wandering through application code.

That separation is one of the central motivations behind Polar.

But, Polar Is a Programming Language

And, here is where we need to be careful.

A policy language that is expressive enough to model real applications is also expressive enough to become complicated.

This:

allow(user, "read", resource) if
    has_role(user, "member", resource.organization);

is easy to understand.

Eventually somebody writes:

allow(user, action, resource) if
    ...

and inside the policy are:

• Recursive relationships
• Role inheritance
• Resource hierarchies
• Group membership
• Tenant boundaries
• Special cases
• Contextual conditions
• Application-specific predicates
• Several helper rules

At that point you have not eliminated programming.

You have moved programming into the authorization layer.

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That isn’t necessarily bad.

In fact, it is arguably the point.

But, it means that policy-as-code still requires software engineering discipline.

Version control doesn’t make a bad policy good.

A .polar file is still code.

Oso Versus Cedar

Cedar and Oso/Polar occupy similar territory, but their abstractions are different.

Cedar centers authorization around:

Principal
Action
Resource
Context

and provides a dedicated authorization language and evaluation model.

Polar is more logic-oriented and gives the policy author considerably more freedom to define relationships and rules over application objects.

Very roughly:

Cedar feels like a purpose-built authorization language with a defined conceptual model.

Polar feels more like an authorization-oriented logic programming language.

Neither approach eliminates the need to model your domain.

They simply put the boundary in different places.

Oso Versus OPA/Rego

The comparison with OPA is even more revealing.

OPA/Rego says: Give me structured input and I’ll evaluate policy against it.

Oso/Polar says: Give me your application objects, relationships, and authorization facts and I’ll reason about them.

OPA is intentionally general-purpose.

You can use it for:

• Kubernetes admission
• Infrastructure policy
• CI/CD
• API authorization
• Compliance
• Configuration
• Security policy

Oso is much more focused on application authorization.

That specialization allows it to provide authorization-specific concepts such as roles, relationships, resource types, permissions, and authorization queries.

So:

OPA gives you the policy engine and asks you to construct the authorization model.

Oso starts closer to the authorization problem.

Oso Versus Cerbos

Cerbos is another particularly useful comparison.

Cerbos is fundamentally a centralized authorization decision point.

Oso has historically been much more tightly integrated with application code, while Oso Cloud extends the model into a managed authorization service.

The distinction is roughly:

The architectures can certainly overlap.

The interesting difference is where the authorization system expects the application data to live and how closely the authorization engine is coupled to the application’s object model.

Oso Versus OpenFGA and SpiceDB

This is where the distinction between authorization logic and authorization relationships becomes especially useful.

OpenFGA and SpiceDB are primarily relationship-based authorization systems.

Their world looks something like:

The authorization graph is central.

Oso can model those relationships too.

But, Polar can also combine them with ordinary application logic and attributes.

So, instead of:

you can have:

That makes Oso less of a pure relationship graph and more of a general authorization programming model.

The Other Trick: Filtering

There is another problem that authorization systems frequently discover the hard way.

It’s not enough to ask:

Can user X read document 123?

Eventually the application asks:

Which documents can user X read?

Those are very different problems.

If the database contains ten million documents, you don’t particularly want to:

SELECT * FROM documents;

and then ask the authorization engine ten million questions.

Oso has invested heavily in authorization-aware filtering and local authorization so that authorization logic can be applied to collections and database queries rather than only individual yes/no checks.

For example, an application can conceptually ask:

Give me the documents this user is authorized to read.

and translate the authorization rules into a database-level filter.

That is a significant capability for application authorization.

Authorization isn’t always:

ALLOW / DENY

Sometimes it’s:

WHERE authorized(user, resource)

That distinction becomes increasingly important as datasets grow.

Local Authorization

Oso’s newer architecture also addresses one of the awkward realities of Authorization-as-a-Service.

If the authorization service needs every piece of application data to make every decision, congratulations:

You’ve created a synchronization problem.

Oso’s local-authorization approach is designed to combine centralized authorization policy/data with data that remains local to the application, avoiding the requirement that all application data be copied into the authorization service.

The resulting architecture can look like:

This is particularly useful for microservices where one service owns a particular piece of domain data but authorization depends upon relationships maintained elsewhere.

The system can combine:

rather than forcing everything into one database.

So What Is Polar, Exactly?

Polar is a declarative logic language for expressing authorization rules.

It has several characteristics that distinguish it from the other systems in this series:

That last line is the important one.

Oso doesn’t really ask: “Which authorization model should you use?”

It gives you a language in which you can express several of them.

The Trade-Off

And, that flexibility comes with a price.

A constrained authorization model gives you guardrails.

A flexible policy language gives you options.

Options are useful right up until somebody uses all of them simultaneously.

You can build: RBAC

You can build: ABAC

You can build: ReBAC

You can combine them: RBAC + ABAC + ReBAC

And, you can build authorization rules that are specific to your application’s domain.

That is powerful.

It also means your organization now has a policy language to govern.

You need:

• Policy review
• Testing
• Version control
• Policy ownership
• Documentation
• Deployment controls
• Observability
• Debugging
• A clear authorization model

The language doesn’t provide those practices automatically.

Neither, does putting the word “policy” in a Git repository.

One More Current-State Caveat

There is a distinction worth making when discussing Oso today.

The original open-source Oso library is currently marked deprecated in its repository. Oso says it is not being immediately end-of-lifed and that critical fixes/support continue, while the project works toward a new open-source release. Oso Cloud is the current managed-service direction, and Polar remains the policy language at the center of the Oso approach.

So an article about “Oso” in 2026 shouldn’t casually imply that the original embedded libraries are the entire current product story.

They aren’t.

The more useful way to think about Oso today is as an authorization platform built around the Polar policy model, with local authorization and managed-service capabilities filling in the distributed-application side of the problem.

Summary

After looking at XACML, ALFA, Cedar, OPA, OpenFGA, SpiceDB, Cerbos, and Casbin, Oso occupies a fairly distinctive position.

It is not primarily: "Let's build a policy decision point."

It is not primarily: "Let's build a relationship graph."

And it isn’t primarily: "Let's create a generic policy engine."

Its basic proposition is closer to your application already contains the objects, relationships, roles, and attributes that determine authorization. Let’s give you a language for expressing the rules over them.

That is a compelling model for application authorization.

It also explains why Polar looks more like a programming language than a policy configuration format.

Because, eventually, authorization stops being a handful of permission checks.

It becomes part of the application’s domain model.

And, at that point, pretending otherwise is mostly an exercise in hiding the code somewhere else.

Usually, somewhere called authorization.js.

Which, six months later, will be 4,700 lines long and contain a function named canUserActuallyDoThisThing().

At least with Polar, you know where to look.

Notes

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