I just went through this, minus the osteo. In some ways, I think knowing he was otherwise healthy made it harder for me. But the arthritis was becoming so difficult and Sam had maxed out on all the medications he was taking--gabapentin, rimadyl, and tramadol. I don't think anyone could tell me when the right time was, I had to decide. It was very hard.
I read a lot of people's experience's here and that was so very helpful. In the end, when he was having really, really frequent incontinence and was struggling so, but his appetite never waned. I wanted to be sure that I could say goodbye to him when it wasn't a pure emergency and I was forced to make the decision. I went with the "better a day early than a day late". We had lots of peanut butter, his favorite, lots of hug and skritchie time, and I took him in on the end of the day of January 28th. I have no regrets. He was not quite 14 and he had graced us for 10+ wonderful years. My time to do him a last act of love.
No one can tell you when to let go, it is a very personal decision. Good luck, you will find a great deal of support here.