First make sure you haven't over filled it with oil "its easy done". Then it depends on where the oil is coming from. Any of the seals such as the crank shaft lip seal then start with the engine breather at the top or some times side of the engine. These can be an open pipe (not nice on a boat) or a pipe feeding back to the air intake. If this is blocked or the filter in it is blocked, the crank case pressure will build up and force the oil out of the seals. If the oil is coming from the exhaust or the manifold mountings then I'm afraid its either the piston rings or valve stems letting the oil past and into the piston bowl and these are more difficult to fix.