Is walking a good exercise for osteoarthritis?

Yes. It can help keep OA at bay. However sometimes even this hurts. Consider stem cell therapies as options if steroid injections or synvisc/euflexxa type injections are not helpful enough. Check out Regenexx.Com.
Yes. Evidence has shown that walking is beneficial for patients with osteoarthritis of the knees, hips, and back.
Great idea. Walking is great for someone with osteoarthritis. First, it's a great form of exercise that will be lightly impactful upon the compromised joints. Second, brisk walking has been shown to half the mortality of elderly people, so there's a significant health benefit for anyone in just briskly walking on most days of the week.
No, resistance train. Is better. For any weight bearing joint OA esp people with high BMI, any lb over ideal body weight puts joint in more strain/stress impact with every stride up to 3-5 times more for hips/knees and 5-7 times ankle base on walking or running. What's going to help every OA Patient the most is to strengthen peri articulate structures like tendon/ligament/muscles for stability, pain contr, joint health.