I was first diagnosed with osteoarthritis of both knees about 15 years ago. What I found stopped the progression was taking a combination of calcium citrate, glucosamine chondritin, and Vitamin D3 (plus a little magnesium). It had initially gotten progressively worse fairly fast (over a period of a year or two); however, after starting on the supplements it has pretty much stopped getting worse and just stayed the same. (Incidentally, this was nothing that my doctor told me to do; I did this on my own!)
Strengthening the leg muscles helps too.
I would be cautious about taking too many NSAIDs. I used to be able to tolerate them in abundance, but now they cause bleeding problems for me and I have to reserve them for only occasional use.