Oil leak
Oil pan gasket
Rear cover gasket
rear main seal
oil pressure sensor
Its fairly difficult to diagnose those top 3 because the block, oil pan, and rear cover all meet at the same junction. Trans has to be removed to change the rear cover gasket or rear main seal. Typically I will give it my best attempt to look between the block and the flywheel and see if you can see oil above the oil pan to block joint. (You can pull the starter to inspect this if its a manual transmission, autos have an inspection hole IIRC). If you can, its probably rear cover gasket or seal. If its below the oil pan to block seem, probably the oil pan gasket leaking.

