If you don't have the exhaust tube at the bottom of the muffler it will catch the oil in the exaust.
You can also just have a small hole it the tube just above the bottom of the muffler to let the oil drain as the engine runs.

I just used a deep hole saw and drilled out the exhaust tube and cut through the guts of the muffler.
I drilled right next the the outlet tube. So I cut out the outlet tube and made the new larger hole at the same time.

This was done on a Ryobi muffler.