Pull the valve cover, each valve has an oil seal to help keep oil from dripping down the valve into the cylinder. Over time, these plastic umbrella seals harden, crack and fall apart allowing excessive oil to splash on valves and leak down into cylinders. A decent mechanic can replace these without having to remove the head from the car, however, if they have been leaking for a while, you likely have excessive carbon build up on your valves which will impact performance. The valve guides may also be worn and then you're pulling the heads for a valve job.
Not ruling out head gasket but there are usually other symptons that will go with that including oil blowing out breathers or coolant blowing out radiator etc.
Could also be something going on with rings but I would start with valve seals and heads and go from there.
