I live in Oslo and have similar mobility issues (Ehlers Danlos syndrome). I am not classified as disabled, but it is hard for me to walk long distances or stay standing for more than about 30 minutes in general. Luckily, almost everywhere I want to go is less than half a kilometer from the nearest bus/tram/metro stop. If I really need to, I can still take a taxi to anywhere in the city center.

All that being said, Oslo is NOT a good city to live in if you need a wheelchair or crutches (which I do, with unfortunate regularity). 500 meters might as well be 100 kilometers if you are on crutches in the winter. Even if where you are going is right next to a train stop, the stops are not very accessible (long ramps way out of the way etc). The curbs do not consistently have ramps at crosswalks. The sidewalks are often covered in ice in the winter. There are stairs everywhere. Even with a car and being able to park near your destination, you still dealt with these issues.

So, I guess my point is, Oslo making the city center mostly car free is not what makes it unfriendly for people with mobility issues. Taking public transportation is still better than parking and walking wherever. But it still sucks if you have crutches.