Holden Commodore: Radiator problem

For the past six months I have had a problem with oil in the radiator of my VX Commodore. I thought it might be the radiator, so I had the mechanic replace it, but that hasn't solved the problem. The mechanic gave it a carbon monoxide test because of bubbles appearing in the radiator coolant. This test indicated exhaust gasses in radiator. Could it be due to problems with either head gaskets, cracks in the head, etc. or timing cover gasket, or water pump problems?