My engine is leaking oil, what could be the problem?

What is your question?

I first became aware of this problem when my engine started to overheat about three weeks ago, thinking I was overdue for an oil change I went to get one and the mechanic said that my oil pan was leaking, after taking his overpriced estimate I went home and when I checked my oil pan out it seemed entirely sound. My engine is nearly covered in oil mostly on the right side (if you would to be standing looking into the engine bay across from the cab), the oil stains go up almost to the top of the engine but not quite (much further than the oil pan {dumb mechanic}). I'm at quite a loss as to explain as to what's wrong with my car but if a reader could shed some light I would be forever grateful. Thank you.