i have a mazda 323 ZM/ZL engen,i found oil in the radiator after changing the head gasket.it once over heated.
I can think of a few things off the top of my head. The first would be a bad head gasket. Since your engine overheated it's possible the head was warped which is preventing the gasket from sealing properly. There could be a crack in the head between the water and oil passages and also there could be a problem with the oil cooler.