Cylinder head gasket, oil pan gasket, oil filter o-ring, oil pressure sending unit.

If it's a fast leak, it's typically from a place where the oil is under pressure. You can wash the area, then watch while the engine is running.

If it's a slow leak you can include non-pressurized gaskets and parts to the list. Wash the area, then add a small bottle of dark purple dye to the oil. After driving long enough for some oil to leak out, you search with a black light. The dye will show up as a bright yellow stain that you can follow back to the source of the leak.