We describe the use of object-oriented programming (OOP) in the design and implementation of TSA, a system for interactive time series and spectral analysis. We show how such features of OOP as inheritance, generic messages, and decomposition of the programming problem in terms of objects have contributed to our goal of providing an extensible data analysis system. We discuss the strengths and limitations of both OOP and the particular implementation we used (Flavors on a Symbolics Lisp Machine) for our particular problem.