We describe an extensible query optimizer for objectbase management systems. Since these systems are expected to serve data management needs of a wide range of application domains with possibly different query optimization requirements, extensibility is essential. Our work is conducted within the context of TIGUKAT, which is a uniform behavioral system that models every system component as a first-class object. Consistent with this philosophy, we model every component of the optimizer as a first-class object, providing ultimate extensibility. We describe the optimizer architecture and how the optimizer components are modeled as extensions of a uniform type system.