You might have an oil pan gasket leaking, but before getting into it, try cleaning the oil with a good degreaser. Oil spilled from changing the oil filter runs back along the top on the pan gasket flange of the block. It can run past the front of the engine by the front crank seal and look all the world like the pan gasket is leaking. I have cleaned many of these up to reveal that there were no leaks, only spilled oil.
