Either your head gasket is shot or your head is cracked. More than likely the latter cause the most common sign of a bad head gasket is white smoke from water going into the piston chamber and loss of power. If your truck overheated to the point of shutting down on it's own or close to it before this happened then you probably either cracked or warped your head. Oil in the radiator is a bit different than water in the oil. There is also the possibility that your timing chain has worn a hole in the timing chain cover, which would allow oil and water to mix (there is a water passage behind the timing cover which leads back to the water pump). This is a very common sign that your timing chain guides have worn down and your timing set needs to be replaced. Hopefully this is your case, as it is alot cheaper to repair than a head repair/replacement.