Metamodeling became in the last decade a widely accepted tool to describe the (abstract) syntax of modeling languages in a concise, but yet precise way. For the description of the language's semantics, the situation is less satisfactory and formal semantics definitions are still seen as a challenge. In this paper, we propose an approach to specify the semantics of modeling languages in a graphical way. As an example, we describe the evaluation semantics of OCL by transformation rules written in the graphical formalism QVT. We believe that the graphical format of our OCL semantics has natural advantages with respect to understandability compared to existing formalizations of OCL's semantics. Our semantics can also be seen as a reference implementation of an OCL evaluator, because the transformation rules can be executed by any QVT compliant transformation engine.