Action Semantics is a framework for the formal specification of programming languages. Two different, recently proposed approaches provide modularity to the framework, allowing for specification reusability and extension. In this work, we analyze the previous approaches, and introduce Object-Oriented Action Semantics, a new form of modular organization of Action Semantics descriptions. Object-oriented Action Semantics does not modify the syntax in which actions are written, the addition of object-oriented features (like classes and objects) is done as an upper layer to the semantic entities and functions. A simple Pascal-like, imperative programming language is described using the formalism. The extension and reuse capabilities of Object_Oriented Action Semantics are demonstrated by adding new features to the description. The semantics of the object-oriented action notation is also presented.