I just had my mechanic tackle this last week. Firestone told the PO it was the rear main seal. My guy sprayed everything down to clear the gunk that was holding my oil in and found it was that O-ring. 20 years weren't kind to it.
He was able to get it drained and replaced in about an hour. Unfortunately I wasn't there when he did it, so I can't comment on how - but he did say it was surprisingly easy.
Haven't leaked a drop of oil since it got replaced! Good luck!