I agree with Jeff. The head gasket is the most likely. When you pull off the head, do not be discouraged if you see a crack between the cylinders. This is common and does not cause a problem. A good head gasket will seal this up and provide dependable service for years and years.
The most common problem is water in the oil and this one is difficult to fix.
The most likely place for oil to get in the water is in the head area. As El Toro suggests, head gasket problems are often indicated if air bubbles appear in the radiator. When the tractor is running combustion gases leak into the cooling jacket. When not running, either water goes in the oil or oil goes in the water.