what are the possible causes for oil in the radiator

Two possibilities.........someone mistakenly added oil to radiator......or your head/ head gasket is bad.

OIL in the RADIATOR is NOT a blown head gasket its mechanically not happenind there. On the other hand it sounds like the radiator has a crack in the oil cooling system. Which is allowing the pressurized oil to transfer inside the radiator. Since its deffinitely not an OHC bc its an astro. Oil travels to the head through the push rods and travels back down by way of gravity through drain holes. There is no area of the head gasket that contacts pressurized oil and if it was contaminating the coolant there then that means a hole big enough to allow oil to freely flow into the coolant jackets would have to be present and if that was the case then there would be a much bigger problem than the present one. I'll bet the coolant used is that garbage orange dexcool $h!t and its turned that nasty dark brown.

What I'm saying is there may not be an issue other than needing a coolant flush.

Wrong

Well i have change the termostat and flush the radiador but now i got oil in the oil pan not radiator? That's it nean i need to replase the heads gaskets?