Oh no, how scary for both of you! Ozzy is quite handsome.

I had a cat who had a calcium build up in the spine due to undiagnosed thyroid issues. Once he went on meds for the thyroid, the calcium build up stopped; did not get worse. Of course, that did nothing for improving the situation. Did Ozzy have any blood work up?

From what the vet wrote, if this is from arthritis you will see some improvement with the bed rest and meds. I hope is it and Ozzy will recover.