I have oil in my coolant reservoir. Could it just be the head gasket or is it almost certainly the entire engine or might it be something else entirely?
Thanks in advance...
The most likely source is the head gasket. There are very few other ways (short of catastrophic engine failure - and that would give other symptoms!) of the two mixing.
Check for excessive smoke (oil getting into the bores), coolant loss (coolant getting into the bores) and a mayonnaise effect in the oil (coolant getting into the oil)
You don't say what car you have, but for quite a lot of vehicles, particularly older ones, that is a do-able job for a reasonably competant diy mechanic. You'll generally need a decent manual, a gasket set & new head bolts, a Torque wrench and a selection of sockets, and make sure to follow the loosening/tightening sequence given in the manual. It is best to get the head checked and/or skimmed by an engineering shop, but you can usually get away without if it isn't too bad a blow.
Following up on @Nick C's answer:
The head gasket almost certainly needs to be replaced and, on some cars, replacing the head gasket is straightforward work with the right tools and procedures.
However, on some seemingly mainstream cars, this is not the case. In order of ease of access to the head gasket, I would rate the common designs as follows:
Inline: relatively easy. A transverse inline-4 is very common in today's cars and all parts can be accessed easily. Longitudinal engine layouts are less common (e.g., BMW inline-6) and you're going to need to reach in to get those back bolts.
V: more work. A V-6 or V-8 is going to be more work, if for no other reason than you have two head gaskets to be concerned about. It's your call whether to try to diagnose each or just replace both.
Flat: cast iron b*tch. I love my Subaru but replacing the head gaskets is not an evolution that you embark on casually. You can see them from the bottom of the car but there's no convenient way to get at them without pulling the engine.