Fast Shafts

I've got a BBC witha remote oil filter. After having the engine rebuilt, I've had a leak where the remote fitting is (where the oil filter use to be). What normally goes bad-the fitting the O-ring or a gasket?? It's very hard to tell exactly where it is leaking due to it being in the bilge. Should I replace the whole fitting? What is the best way to solve this? New Fitting?? Thanks in advance!
Bob G