Mark,
2, 3 and 12 is a good place to watch the action. 12 has an embankment behind the green that is great for spectator view that would be a good starting point. From there, you can easily get over to 2 green, 3 (which is the best par three on the course) and 13 tee. While the sight lines are not as good, 14 is a fun hole to watch, as it is a short par four that is well within driver distance. I also like the outside of the dogleg on 18, where you can easily see tee shots, approach shots and putts.
As an aside, I will be around Oak Hill all of PGA week. Anyone who wants to hang out or meet up at the tournament should send me a PM.