Hi Harry,

It depends a bit on your landscape goals, but in general, plan for the trees' mature size (or plan to thin them).

Red oaks tend to have rounded crowns that can be up to 40 feet wide; I'd recommend 30 feet as a minimum distance between trees.  Be sure to adjust the number of emitters upwards as the trees grow, and move them away from the trunk.  Water at the edge of the canopy or slightly beyond it once the trees are established.

I hope this helps!

Cordially,

John