We show how modern proof environments comprising code generators and reflection facilities can be used for the effective construction of a tool for OCL. For this end, we define a UML/OCL meta-model in HOL, a meta-model for Isabelle/HOL in HOL, and a compiling function between them over the vocabulary of the libraries provided by Featherweight OCL. We use the code generator of Isabelle to generate executable code for the compiler, which is bound to a USE tool-like syntax integrated in Isabelle/Featherweight OCL. It generates for an arbitrary class model an object-oriented datatype theory and proves the relevant properties for casts, type-tests, constructors and selectors automatically.