I'm so very sorry to hear this, Laura, and can identify completely with the shock, anger, and grief you are feeling. I've lost two dogs to cancer, not osteo, but hemangiosarcoma. I didn't face the decision re: amputation, but I fully understand your feelings and believe it is an individual and personal one to make.
Unfortunately, there are a lot of people here who have had experience with osteo and I'm sure they will offer their advice about pain medication and what to expect.