I'm going to give general tips below, but for more specific help, if you tell us your exact limitations, others can be more helpful. You say you use oxygen - is that all the time, or just sometimes? Can you climb stairs not at all, or only a few, or just one flight but not more? Are you mobility impaired in any other way?
To give just one example of why these questions matter, if you can't walk up or down a flight of stairs, many restaurant bathrooms will be off limits to you. Many restaurants are at ground level, but their bathrooms are often up or down a flight.
Here's Rick's page on the subject of accessibility in Europe: https://www.ricksteves.com/travel-tips/trip-planning/travelers-with-disabilities
Note that he used to have a book on the subject which is out of print. I was going to recommend buying a used copy, but I see he has it posted online for free, here: https://www.ricksteves.com/watch-read-listen/read/easy-access-europe
Here's some earlier threads on the subject (different needs from yours, but lots of useful links):
https://community.ricksteves.com/travel-forum/france/wheelchair-accessibility-paris-france
https://community.ricksteves.com/travel-forum/to-the-west/paris-wheelchair-accessible-apartment
