Actually, there are a few more places...
On the front of the engine there is usually a pipe thread plug fitting for the oil galley. It can leak as well. Sometimes they aren't tightened and sealed properly after a rebuild when that particualr fitting is used on initial runs.
As mentioned in another thread the seam of the engine can leak, too. And if you happen to have a front mounted prop governor, those seals can leak. The governors usuallyu need to be really tightened down hard to prevent leaking.
Vic