Paul and especially Marc:
The overflow tank is located in the right fender behind the wheel. Yes, the inner fender trim is removed to get to it.
Marc, you or your mechanic keep referring to the pressure tank as the overflow tank. It's important to keep this straight. The part you can see under the hood is the pressure tank, it has a pressure cap (or under common terms is referred to as a "radiator" cap). Under the pressure cap is a fabric wrapped hose which attaches to a nipple on the filler neck of the pressure tank, kind of leads off into la-la land, looks like it probably leads to the ground eventually, right? Nope. THAT hose is going to the "overflow" tank, the one everyone forgets about, out of sight, out of mind.
If you get a bad case of blown head gasket, you can get quite a bit of oil into the cooling system. The extra oil fouled coolant will eventually make it into BOTH tanks. If the problem is fixed with the head gasket and everything cleaned up and the PRESSURE tank cleaned or hopefully replaced, but no one removes and cleans or replaces the "overflow" tank, then the cruddy oil will eventually be introduced into your nice, clean cooling system through the normal way of the coolant contraction during cool-down periods, as it is intended to work.
Marc, I am not suggesting that the oil got into the cooling system through any normal "servicing", maybe this is just a mis-statement. I would be more curious if the head gasket had failed and was repaired, or even more curious if the previous owner also felt that the head gasket repair was imminent and he bailed on the car. If it were mine I think I would REPLACE the pressure tank, it is too complex inside to do any real good cleaning, and remove and pressure wash the overflow tank, it is pretty simple, although you may consider getting a price on a new overflow tank as well. Usually if the head gasket is "really" blown, you'll know it, the coolant will really look bad, not just the inside of the pressure tank either, you'll have "beige" looking coolant, kinda like butterscotch pudding, "without all the calories!".