We ask that you tell us what engine you have. It makes troubleshooting a lot easier.
If we're talking about Chinese 2 stroke engine and you have oil inside the magneto housing then you have a bad crankshaft seal. You'll need to remove the magneto rotor (magnet) from the crankshaft to gain access to the seal. Mark the magnet so you put it back exactly how it came off and order a new seal from one of the sponsors here on the forum.
Tom