Well, it could be the Valve Cover gaskets, a front main seal a rear main seal an head gasket or one of several other gaskets. It could also be a cracked Block or Head. Or it could be your oil filter has come loose.
As you can read there are quite a cuases of an oil leak. Now you will have to try and narrow the field as to the exact gasket or gaskets. And to do this you will need to degrease the engine. Then let it run for a while, to make it easier you can go by your local parts store and get a oil leak detection dye. You will also need a black light. It will also help to do it in a garage with light out or dim, if no garage then do it at night. Ad the dye run vehicle for a while then turn engine off take black light and look for florescent green dye and that will be your leak.