I'm so sorry. I've been through this three times.
My feeling about this is that osteo is an extremely painful disease, more painful than any of us would want to experience. My vet told me that if a dog looks like she's in pain or discomfort, we can be sure that she is in a zillion time more pain than she lets on. I tried my hardest to let mine go before they reached that point, but I'm afraid that I failed them.
Good luck. You will know what to do.