I completely agree with you about the Roundup, etc. I doubt any groundcover would harm your oak unless it is already on its last legs, but why don't you use landscaping fabric or heavy cardboard over the weeds and mulch underneath the oak to the drip line? The dripline is the area below the outer circumference of the tree branches. Water drips from it sort of like it does from an umbrella. The drip line area is also called the Critical Root Zone (CRZ) or Root Protection Zone (RPZ).