You cannot combine two separate itineraries, period.

I do this all the time. I'd leave a minimum 3 hours at SFO in both directions. The outbound is the riskier of the two, since you risk the ticket if you miss, so you might consider an even longer "connection" (it's not really a connection in the technical sense of the word, but it's still the best descriptor). If you fly UA DFW-SFO, they will check your bags (if any) through to OZ. (Note though OZ will likely be leaving *A at some point assuming the acquisition by KE is approved.)