Typically, oil leaking on the Y-pipe is due to either the rear valve cover gasket leaking or the rear cam seal leaking. It could be either of the 3 cam seals or the front main seal as suggested above.
If the rear cam seal is the problem you will soon start to have cam sensor problems too. I suggest getting under the car with some degreaser and start spraying it down so you can pinpoint where its coming from. The valve cover gasket is a lot easier then the cam seals. If you are going to do the cam seals you will want to do the front main, water pump, and crank sensor as well.
Once you get these done it will be years before you need to worry about them again.