1996 Buick Roadmaster Oil in Water

Hi: I have oil in the radiator overflow tank on this car. Did oil change and checked oil first and was not showing on dipstick. Checked farther and found that overflow tank had lots of thick oil residue in it. The transmisssion fluid level is OK. There is not radiator cap on these cars so can't look directly in radiator but that's where the overflow comes from. Also have 96 Impala which uses the cooler in one side of the radiator for an oil cooler but Bucik does not have an oil cooler. It uses both sides of the radiator in series as transmission cooler. Found one web site that suggested head gasket was issue, but don't think that there is pressurized oil in the head. Seems to leave the engine block itself. Figure it has to be oil under pressure to get past pressure in the cooling system? Any ideas? Also need to know how to prove diagnosis if there are ideas.

Thanks!