Very likely a head gasket. About the only other possible to check out is whether or not the car has any kind of “oil cooler” incorporated into the radiator. Some cars have oil coolers and trans oil coolers and to save weight and space they put them all in one unit. If you have this kind of system oil could be leaking from the oil cooler tubing into a regular coolant tubing due to rust, or leaky “0” ring seals. Look carefully at the radiator for lines coming into and out of it. There are lots of lines in the radiator area; AC lines should be easy to identify then look for more lines which might carry oil or trans fluid.