Hi.
I had this problem once with my car. Basically some water ed the oil, but not enought to knacker it. The falult lay with a crack in the oil cooler, so I bypassed it. Disconnect the two pipes entereing into the oil cooler and place a short metal pipe inside each end, push the ends together then using jubiliee clips tighten.
Presto, should prevent any oil into the water. You'sll have to flush your radiaort and other coolant pipes out with loads of water, preferabllly from a hose to dislodge any brown gunk, but i've had no problems with mine since. Also maybe good time to invest in new radiator and watter pipe that enters into the side of the cylinder head from the out pipe of the heater matirix.
Hope this helps.
Green.