How many feet should I clear around oaks?

It's all about the sunlight. Simply cut the ash as needed to get the oaks the majority of the sunlight. the ash will grow much faster and will stump sprout so don't be surprised if you have to address this more than once. Try to make access to the sun even to prevent the oaks from leaning. Something you have to consider is if you want the trees to grow up or out. If you leave some competition around the tree it will tend to promote upward growth where if you remove that competition the tree will send energy spreading it's canopy - which you may or may not want.
Emerald ash borer not to you yet?
Crop trees should be allowed enough light on their edges to expand. But its a deep subject. Too much light and the crop tree may grow out instead of up.
One thing you must avoid is the top of your crop tree losing its space in the forest canopy.
There are a lot of resources on the web, including Youtube, to help you.
I try to give my favored trees at least three sides with room for the crown to expand. Cutting extra competing trees gives more room to expand but exposes the favored tree to more wind. Blow overs are a big deal here.

With ash values being fairly high at this time for good ash, another question is how should you release the oak trees?
Should the ash be cut by loggers and sold, girdled or hinged or simply dropped and left. Skidders damaging roots of saved trees and possibly scarring up trees is always a risk when selective cutting. Skidding only on frozen ground would certainly be a consideration for me as would opting for smaller skidders and not hauling out several whole trees at once. It is pretty much a given though that the ash trees will die on their own soon and maybe no action related to removing Ash trees might be the best action for your oak trees. I'd be surprised if the ash are not already showing signs of weakening-ie, shrinking crowns, less leaves and peeling bark or bird peck holes.
The Borer is around and most likely on my property. I will be speaking with a local logger about select cutting in a series of yearly cuts. My main reservation for not having my trees logged has always been scarring the property. I'm in a low lying area and frozen skidding is the only way I would consider it. I like the idea of smaller skidders if that is an option with this company. Appreciate the thoughts!