Flat wasn't invented when they put Oakmont together. There's more up and down there than a Coney Island roller coaster and it's critical you take that into account when playing.
A quick and easy way to get close to the right club selection where elevation is concerned is to convert the elevation change, shown in feet, to yards and add or subtract that from your distance calculations.
For instance, you're looking at a 136 yd approach to a green 37 ft above you. 37 ft is 12 yards (always round down for consistency) and since it's above you you would add it to the 136 and shoot for 148 yards. Conversely, if the hole was below you by 37 feet you'd be hitting to 124 yards.
This technique will get you to a place where you can take your own methods and refine them. The key is to know that your club yardages are based on flat terrain and there's no such thing at Oakmont. GL