Oil leak can't figure out what part it is

First check that the oil filter has the gasket in place and isn't clogged with dirt around the seal. Then make sure the filter is screwed all the way in.

The best way to pinpoint the oil leak is to clean the surfaces thoroughly with a solvent and rags, then run the engine for awhile until the leak reemerges. You'll then be able to track it along the freshly cleaned surface.

If it's behind the filter, then it's likely an oil cooler and they have replaceable gaskets to solve that.