Oso is an open-source framework for adding fine-grained authorization to applications, using a declarative policy language called Polar to express role-based, relationship-based, and attribute-based access rules directly alongside application code. Rather than scattering permission checks across if-statements, teams model who can do what in one place and query it consistently, with official libraries for Python, Node.js, Ruby, Java, Go, and Rust. The original open-source library is now in maintenance mode as the company focuses on its newer offerings.