Does it have motor separate that has a belt driving a pulley on the compressor? I think that may be one that does have pistons and there would be some oil, more as it gets older. I doubt that if it is relatively new that there is a significant amount. You can get oil and water separators that remove the oil and water from the air (folks that paint cars need clean air, any oil or water in their compressed air would ruin the paint job).
I doubt that you need one, but if you see any oil film around the bubbles, it may be some oil in your air....
You just put them on the output of the compressor before it goes to the pond.