Corsa cdti sri 2004 oil in water

[simonx50]

Hi Guys, as the title shows I have oil in the water...the whole contents of the engine oil full to the brim in the header tank!
Been told it could be a head gasket but likely to be oil cooler, just wondered if any one has experienced this and could give me some advice as to removing and getting all the oil out of the cooling system.
Can any one shed some light on how the cooler works and why it would of done this?
Cheers!