What did your doctor recommend ? Your doctor should have given you some guidelines as far as what you could and couldn't do with the osteoarthritis in your knees. Did they say you could do what you wanted as long as it didn't hurt ? or did they tell you don't do X, Y or Z ?
You could ask your doctor if swimming or water aerobics is an option. Being in the water helps take the stress off a person's joints. Another possible exercise would be chair aerobics. If you can't stand for long periods because of your knees, sit in a chair and do upper body strength exercises with hand weights or resistance bands. Coach Nicole also has a short 10-15 minute chair workout posted in the fitness section.
But, ask your doctor first whether or not these could be good options for your knees. Because we don't know anything about you, none of us is really qualified to make suggestions.