We present a formal semantics for an object-oriented specification language. The formal semantics is presented as a conservative shallow embedding in Isabelle/HOL and the language is oriented towards OCL formulae in the context of UML class diagrams. On this basis, we formally derive several equational and tableaux calculi, which form the basis of an integrated proof environment including automatic proof support and support for the analysis of this type of specifications. We show applications of our proof environment to data refinement based on an adapted standard refinement notion. Thus, we provide an integrated formal method for refinement- based object-oriented development.