About three months ago, I arranged for and flew with a family member who required oxygen during flight. You will have to contact the UA Aeromedical department, provide them a medical prescription that specifies the oxygen flow rate required, and pay $100 per flight. We flew SFO-LHR in C on a 777. The oxygen tank was quite large and heavy for that long flight and was mounted in the overhead compartment with a tube duct-taped to the ceiling / wall for passenger comfort. The technician who installed the tank was very helpful and even flipped the heavy tank around to provide a longer length of tube so it was comfortable even with the seat fully reclined. Only certain seats are suitable for oxygen and the GA did some polite juggling of seats in order to get us seated in the appropriate seats. For the amount of special attention that it required in organizing, installing, and FA monitoring during flight, I thought the $100 cost was more than justified.