use of metro system with need to carry oxygen and difficulty walking up stairs

I think you will experience more of the charm of being in Paris by bus or cab than by using the metro since you will be able to see Paris as you are travelling. Don't put extra pressure on yourself by putting yourself in a position that is questionable with the metro.

And your memory is correct. Most Métro stations do not have escalators. Some RER stations have escalators, mostly only in the UP direction.

With buses you enjoy the scenery. The metro is convenient, but hardly charming. Embrace buses. Also the batobus (boat bus) would have far fewer stairs than the metro. And you're on the river, so that's fun. There is a stop by the Eiffel Tower.

Yep, ride the buses. You can buy the booklet "Paris Bus - 109 Lines" for about 7€ at a newsstand. It has a 2 page map for each bus line, showing all stops and transfer points. The secret is to be patient - buses do get stuck in traffic and can make unscheduled route changes, so best not to try to pack too much into one day. Have a wonderful time.

As others have stated, most of the Metro stations do not have escalators or elevators. I don't recall the steps being especially steep and I'm sure you could manage if you took it slowly, even with oxygen. However, you'd have to stay on the side to avoid being bowled over by the hordes moving past you at a much higher rate of speed.

If you ever feel it would just be easier to catch a cab a taxi stand, you can "get home" from most in-city sightseeing for about €10.

Actually, here is a free Paris bus map.
http://www.ratp.fr/informer/pdf/orienter/f_plan.php?loc=reseaux&nompdf=bus_paris_geo&fm=gif

I agree with getting the book recommended by Carolyn. It is in French but the maps and lists of stops will be completely understandable. The free bus maps online or at stops are difficult to use, as firstly the colours used for the lines are quite similar, and second there are many locations in Paris where the same route takes two different streets for the two directions of travel, which can be very confusing on the official map.

IME the only legible official transit map is the Plan 2 map available at the metro info/ticket booths. It has the metro lines on one side and bus routes on the other. It is worth picking one up. I found it to be useful used in conjunction with the booklet. If you want to experience riding the Metro just because you are in Paris, try Line 14, which is one of the newer lines and I understand it is completely handicapped accessible with escalators and elevators. On other lines, even if there are some escalators, they are not at all stations and may not serve all levels or entrances. Also, keep in mind that once in the Metro system, there is a lot of walking required to get to the trains, especially when transfering between lines.

I am handicapped and use a scooter. I always take the bus and then ride my scooter where I wanted to go.