I had exactly the same with my car. It started out with some loss of coolant. Never had that before. I topped it off, and went on. After a couple of miles my oil light came on in a tight turn. stopped immediately, and looked under the hood. A brown "mess" boiled out of the coolant reservoir. I knew immediately that it was coolant mixed with oil.

So my car was towed to the dealer (off. Volvo), and I discussed the possibilities with the tech manager. In my opinion there were three ways oil and water kan mix together:
1. In the turbo
2. the radiator (oil cooler)
3. the head gasket.

While the car was running fine, and did not smoke, the turbo felt off.
There was no oil cooler on my car according to the tech manager, so he counted that off too.

Soo, the only possibility left was the head gasket. I knew that Volvo 5 cil head gaskets hardky fail, so did some research about it. After all there was a oil cooler in my car. The dealer stripped the head, and replaced the gasket. Under while I attended the dealer to my cooler. It is a oil cooler which is integrated in the radiator. Now after all the problem was related to the cooler, but the fact that your radiator is replaced, the problem might be related to your turbo. look if there is smoke from your exhaust, or if your rear bumper is greasy. That indicates a turbo failure. Probably the former owner replaced the radiator, and came to the conclusion that it was the turbo, and sold the car. Be carefull to strip the cil. head. From my own experience and a lot of others that is hardly never the problem.