If engine oil, check:
1. Oil sump. If it leaks you have two choices: a. Seal the leak (usually at the plug) with aluminium welding) b. Replace the rubber plug/seal
2. Gaskets (lower engine side)
If tranny oil, check:
1. Flywheel oil seal. Cheap stuff but make sure you get the right one. Find a mech that knows how to fit it properly. The labor cost is a bit high because you have to bring down the gearbox
E46 leaking oil? Kinda early for gaskets to leak deteriorate, rite?
Btw, the colour of the oil will give u also a clue where it's from....darkbrown/black from engine, gearbox......normally light brown, brake fluid...clear etc.