oiler for an engine driven compressor

can someone tell me what type of oiler i need for an engine driven compressor, and where i can get one, i know it is more than just an oiler, i just cant think of the name right now, any help would be appreciated

thanks

I picked up a craftsman air tool oiler from sears only because it was 60% off. I think any inline oiler should work.

is that all i will need or is there an oiler/water seperator? i have a water tarp from my air ride is that the same thing?