In the last years an increasing number of companies have shown interest in applying verification techniques in model based software engineering and Océ is one of them. To apply such techniques it is necessary to have a formal semantics of the modelling language used, which in our case is OIL (Océ Interaction Language). We show the basic and important ingredients of OIL and give their semantics. We also show how the formalisation of these semantics, using a transformation to mCRL2, is realised.