I have a 89 Cherokee and have been trying to track down an oil leak. After eliminating several possibilities of where it is coming from I move to check the rear main. After removing the starter I noticed a hole the size of a quarter in the rib above it. Any ideas on what this hole is? Could it be a cause of an oil leak?

Well, unless you tossed a piston and blew the side of the block out, that hole should just be for the starter. Just to be sure, did the starter cover the hole when it was attached? If this is true, you would see a large toothed gear (Flywheel) inside the hole.

Also, the type of engine would help, Is it a 4.0 Inline 6?

It's a 4.0L I6. I knew about that hole that goes to the flywheel, this one is directly above the starter, I can push against the dust shield and go up a little ways, I didn't have a very long screwdriver at the time. I felt around on my 97 parts keep and I feels like there is something above the starter but I couldnt see. Btw my jeep runs fine it just has a slight oil leak.