I think you still have to specify what you think would make your flights enjoyable.

Video-on-demand entertainment system? Power at the seat? Free alcohol? Extra legroom (already answered)? Better food? Nicer airports for connections? Fewer connections? Stopovers in interesting places?

That said your question might be hard to answer because everyone is familiar with their favourite airline and FF programme and not so familiar with the others.

i think your only options for a one-stop flight to OSL are UA, SK (both through EWR) and DL (through CDG or AMS). The first two would give your PHX and TUL sisters a one-stop trip there through EWR but require an extra connection for everyone from BGO. Whereas DL would require an extra connection on the way there from PHX and TUL but no extra connection to get to BGO.

I suspect SK is the most enjoyable of the three. I haven't flown in economy on them, just premium economy. But the service was pretty good. I think they may provide free alcohol for example. The planes can be a bit older though and you would have to check about the fuel surcharges.

Delta versus UA will presumably bring out the partisans for both sides... i would check which type of aircraft they each fly to TUL and PHX with. ERJs are ok but I would avoid long flights on CRJs and other smaller aircraft.

Icelandair also flies to MSP and OSL but I assume they're not partners with any of your programmes... They would make a very different stopover.

If you don't mind extra connections there's also AC with newer seats, power, entertainment, and alcohol. But imho it's pretty hard to justify extra connections.