If its coming from the oil filter - just make sure its actually tight (you might be that lucky).
As DH says, clean it all up, get the engine running, then get under it.
If its on the oil filter and no higher on the block, its probably the oil cooler - be it loose from the block or damaged.
If the oil is higher up on the block - then its probably the rocker cover, but I wouldn't have thought enough could leak out from there, all the way down the block to form a puddle on the ground, without building up somewhere else first.
If its leaking at the sump, the sump gasket takes about 30 mins to change and will set you back £8, get the MK3 rubber/metal one. It could be the old owner has done an oil change and not replaced the crush washer and its leaking from there - 20p for a new one.
If it was a puddle - I wouldn't drive the car until its sorted.
