Might be the front crankshaft seal that's leaking. You have an engine problem but didn't bother to list the engine size so anything is a guess. If possible, look around the harmonic balancer to see if oil is sprayed out from it. That indicates a leaking seal. If the area is just wet with no spray pattern, look for the highest spot you see oil. Oil running down from up higher will puddle on the lip of the oil pan.
Another trick is to add a small bottle of dark purple dye to the oil, then search later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source.
If it's a big leak you might find it with a smoke machine. Inject the smoke into the dipstick tube, then watch for where it sneaks out. If nothing appears, the leak is most likely from a place where the oil is under pressure when the engine is running.