If you had a head Gasket problem, you would generally have over heating issues and noticeable coolant consumption. The oil cooler would not cause the engine to over heat, unless you ran the coolant really, really low and cooked the motor. Have a Block Check performed, which is a chemical test of your coolant to see if your head gasket is leaking internally. Also find a shop who has a hand held Mirror, so they can tell you PRECISELLY what and where it is leaking. I just worked on a Catera this week, and you can use a Mirror to isolate the leak, if in fact it is an external leak, trust me.