That rubber seal doesn't prevent oil from coming out - it's there to prevent dirt/water from getting in.
Andyb is correct - it could be the crank rear seal if it's engine oil coming out of the wading plug hole, sump seal (RTV), or maybe the crank rear main bearing T-seals leaking.
T-seal is major work, crank rear seal is also almost as bad major work, and the sump seal is very messy, but not too bad to do.