As its a turbo engine it will have an oil cooler and as you have said it probably is leaking. Disconnect the water pipes going into it and join them together with a small piece of copper pipe. run the car for a day or so and see if the problem goes away . If it does you will need a new cooler as the oil will overheat without one. If its still putting oil into the water obviously its not the cooler.
You may need to flush out the coolant tank with detergent, Daz is good in hot water to kill off the oil to know if its still coming in from the cooler.