are you shur it is motor oil?sounds to me like it is a leaky heater core.the only way you could possibly have oil in the car is if it is refrigerant oil from your evaporater if you have a a/c car.
if it turns out to actually be motor oil, mine did this on the driver's floor. the mechanical aftermarket oil pressure guage hose sprang a leak. its plastic. i replaced it with a copper one.
Do you have an oil leak out front at the firewall/oil filter location ? If not it is probably antifeeze from the heater core leaking like 69firebirdman said.