I flew recently ICN-SFO-ORD-SFO-ICN.
I needed oxygen service on each leg. I made arrangements in advance, and UA handled it well.
There are only certain seats you can sit in with this request, and it sometimes means that they may have to move someone from their preassigned seats in order to accommodate me, and my traveling companion. I am sorry about that happening. I usually try to book well in advance so I can get the required seats, and not have to disrupt anyone.
The flight attendants looked after me, and I didn't have a concern. ^^^