the only way that can happen is oil in the combustion chamber. You'll need to do a compression test and check for leak down. Say it reads 200 leave the gauge on for 15 min. then come back if you're still reading 200 that is good. If it doesn't. The piston rings are letting the oil blow by them that's how it gets in the exhaust.

can happen by overfilling engine oil..... check oil level.