The definition of an oracle function for model transformation is challenging because of the very complex nature of models resulting from a transformation. Validating the correctness of an output model requires checking a large number of properties on the structure and semantics of this model. The oracle function can thus be very complex if it checks every property. In this paper, we identify and discuss important issues that must be tackled to define model transformation testing oracles. We also propose several oracle functions and analyze how they take advantage of different model driven engineering techniques.