If that oil is coming out from between the cover plate and the bell housing then it is likely the rear main seal if it is engine oil. It could also be the transmission front seal if its not engine oil.
If it is running down the front of that cover then moving on to the bell housing then it may be the oil pan gasket.
If it is running down the front of that cover then moving on to the bell housing then it may be the oil pan gasket.