I lost my Kimmie last year to osteo under circumstances such as those that you describe. She had been diagnosed with OS 27 days earlier and we were taking her to the vet for a scheduled routine X-ray to evaluate the aggressiveness of the osteo. Unfortunately, her leg broke (in two places) in the car as we made the first turn off our street headed to the vet. It was the absolute most horrible 5 minute drive we have ever taken. My girl was extremely stoic for the 8 years that I had her. She had never cried, never flinched, through medical tests, blood draws, injuries requiring stitches and drains. But she screamed the GSOD the entire ride to the vet's office with the broken leg. We put her down on the x-ray table so we wouldn't have to move her again and cause any more pain.
It is something that will haunt me forever. If I had it to do over again, I would have let her go much sooner and not had her suffer those last few minutes. If I ever get another OS diagnosis, we will do weekly x-rays and I will do everything in my power to prevent a break.