Water in the oil - Cars & Trucks

Didnt specify vehicle but --possible damaged radiator if has oil cooler built in ---most common is a blown/damaged head gasket ---blown /damaged intake gasket=or intake manifold --warped /damaged head ---damaged piston,piston rings,and or cylinder wall -----need to pull the plugs and look and see which is fouled and do cylinder compression test the following link shows how to read a sparkplug
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Radiator coolant in the engine oil is normally an indication of a failed cylinder head gasket or a crack in the cylinder head or both.

Running the engine in an overheated condition will cause head gasket failure and often a crack in the cylinder head.

The problem you have is coolant is escaping into the engine's lubrication passages via the failed head gasket or head crack and contaminating the engine oil. If not fixed you will cause further damage to the engine as oil contaminated with radiator coolant cannot adequately lubricate the engine.

Running a leak down test on each cylinder will confirm the leak. In this case you are going to need to have the cylinder head removed and thoroughly checked for cracks and any warpage as well as a new cylinder head gasket fitted. The condition the engine block surface will also need to be checked.