Without knowing the history of your car, it could be an oil leak or someone may have just spilled oil on it while filling the oil. Is your oil ever low? Your best bet is to clean off all the oil stains with some brake cleaner anywhere you can find it. Then drive around and check it every other day or so to find where the leak is coming from. Hard to source a leak if everything is already gunked up and oily. If all else fails, you can buy an oil UV dye kit with a UV flashlight. Drive the thing around and then shine the UV flashlight in the engine bay in the dark. If there's a leak, you'll find it.