Prof. Heinrich Hussmann held a lecture about OCL and other formal specification languages  at the Technische Universität Dresden during the winter semester 2001/2002.  The lecture builds a bridge between practically used specification notations from software engineering (like the Unified Modeling Language UML) and pure formal specification approaches based on mathematical logic. OCL is still presented in its  first language version that only supports invariants and pre/post conditions.

You can find the website of the lecture including slides and lecture notes under http://www-st.inf.tu-dresden.de/Lehre/WS01-02/fs/.