Holden Captiva - Lots of oil pouring out the radiator expansion tank

So, you've got a leak between the cooling system and your oil passages. My best guess would be a head gasket failure. As you might imagine, this is not a good situation, nor one that you can live with for any length of time.

Off hand I can't think of a way to confirm that it is the head gasket without pulling the head and looking. So, before digging into the head it would be worth checking to see if there are other places where oil and water get close – for example an oil cooler.

However a quick search seems to turn up mostly issues with the head gasket.

howdyyawl! if the presence of oil is in the coolant and coolant in the oil, check the filler cap. remove the cap, if white residue is visible, there is definitely a leak in the gaskets somewhere. maybe in the head gasket or timing cover, in any case it will be an expensive fix, especially if a diesel. If an oil cooler is fitted, check all points of coolant and oil fixtures points. Check all oils and fluids don't have any restrictions, this will cause cross leakage. Hope this info is helpful.