my calibra v6 had this and the garage i bought it off of thought it was the heads but my local mechanic told me the oil cooler goes a lot on v6's and it turned out that was the problem a lot cheaper than taking the two heads off dont no if the omega has the same oil cooler but i would imagine it does i would check that out first
Yeah, check the oil cooler.
It is submerged in the coolant in a slot between the heads.
Getting to it on a Calibra is fairly straightforward, you have to remove the inlet system, injectors etc etc to get at it. Its not hard, just fiddly.
I cant remember the inlet setup on an omega but it will require a similar amount of dismantlying I would reckon.
The cooler itself costs about £160 ish from Vaux.
If you are getting a garage to do it, budget for several hours labour changing it.
A few people have just disconnected the pipes to it, leaving it in place but unused and fitted an aftermarket external oil cooler instead. This works out a lot cheaper, but you will need to find oil hoses with suitable fixings to plumb it in. Your local Pirtek/Hydrasun kind of place should be able to make them up if you cant find them anywhere.