Oil in coolant

I own a Holden Vectra 2001, CD sedan 2.6 V6. It has 160 000 k's on the clock.

I have oil in the coolant. The mechanic has pressured tested the heads and replaced the gasket and the problem still exists. His worst case scenario is I have a crack in the engine block. Could there be any other reasons as to why this is happening :shock: