Granted that you have a problem figuring out the source of the oil, I won't address that (but suggest that the least problematic source is the radiator's internal ATF heat exchanger, assuming the "oil" is actually ATF; the alternative, blown head gasket, is a much worse scenario).

But as for cleaning out the oil from the coolant passages throughout the engine, the best is a treatment you can get from a local Mercedes-Benz dealer -- it's what they use in their shops (and you know how fastidious, cost-no-object they are), that I learned from my days as a multi-MB owner:

First step: coolant passages degreaser, MB p/n A-001-986-21-71.

If you then want to remove scale (mineral) buildup, follow the above with this citric acid treatment: MB p/n 000-989-10-25

Both of these are safe for your Volvo -- MB engines are basically the same combination of alloys, and even specifies the same special antifreeze, a Glysantin-based solution (MB dyes theirs yellow, while Volvo dyes theirs blue, but otherwise they're the same chemically).

Good luck.