The typical cause of this issue is the headgasket.
The headgasket has oil and coolant that pass through it from the engine block to the cylinder head. If the head gasket fails between an oil galley and a coolant galley you can get the mixture of oil and coolant in your radiator. As well, the issue can manifest itself in your engine block where you can have oil and coolant mix in that area of the engine.
The cause of the failed head gasket could be a situation where the engine was overheated. This can warp a cylinder head. If that is the case, a shop with appropriate equipment could take a few thousands of an inch of the head and make it flat in order for it to have a good seal against the engine block.
In order to repair your vehicle, you will need to replace the head gasket by removing your cylinder head, replacing the gasket and putting the head back on.
This is not a trivial repair. It requires quite a bit of work and knowledge.
Best of luck.