Compared to Winged Foot and Shinnecock, I actually found Oakmont to be quite good from a spectator's perspective. The lack of trees allowed viewing several holes at once, from certain vantage points. From the top row of the grandstand behind 13 green you could watch the 2nd and 3rd shots to 12 and the approaches to 14 (with binoculars). The lack of trees also meant it was hot, hot, hot! The most difficult viewing was on holes 2-8: too congested, and too close to the main spectator entrance. We were there on Saturday and purposely high-tailed it to the clubhouse side of the PA Turnpike for more breathing room.
There were far fewer corporate tents (it seemed) than at Winged Foot. Despite having 36 holes, WF was claustrophobic with tents. I'm not sure if that is a function of tight quarters at WF, or because more tents were sold (it being NYC, after all).
The home hole at Oakmont was far superior, spectator wise, to the 18th at WF. Oakmont, by the way, was also much hillier than I had imagined it to be. It is a magnificent piece of property.