If one has initial stages of oa in the hip or knee, is it inevitable that hip or knee replacement surgery will happen? how can i avoid it?

Not inevitable : Hip and knee OA are very frequent and is OA in the weight bearing joints, thus weight loss and exercise to strengthen the muscles surrounding this joints is key. The x-ray can showed bone on bone OA but what matters is if you are having pain. If pain and had tried lose weight and PT, then try medications first, even injeccions. If nothing works then consider surgery if your lifestyle is affected.