Oil & water don't like each other. So put a little liquid detergent (preferably Tide) directly on the oily spot (no water) and work it in by rubbing. Then wash per usual in the machine.

Is more effective if stain not previously washed/dried in (but still worth a try regardless). Also, by oil, I mean food oils/grease. No idea if it works on motor oil.

Learned it from the mom in the household I stayed in when in Spain twenty years ago. It has saved many a garment!