Head gasket would definitely be one of the first things I would check, especially given how easy it is to change a head gasket. If you want to do a little more checking before ripping the head off, you can pull the spark plugs and check for water on top of the pistons. Hopefully
there's no major damage to the valves or any thing else. If your lucky it's just a small leak and the water/oil have been slowly mixing. Also to note, you may want to pull the oil pan and check the crank mains/rods for any sign of rust. I'd go ahead and pull the oil pan anyway to make sure I get all the oil/water mix out of it before I even put new oil back in it.
Anyways, good luck, and I hope it's nothing more than replace a head gasket and oil change to fix it.
