I am so sorry!!!
I would call the O2 company and throw a fit and have them reimburse you...the guy quit, and no, that was not their fault, but they are ultimately responsible for keeping tabs on their employees. It would be like if I suddenly walked out on my patients, it wouldn't be the hospital's fault but the hospital would have to find my replacement... (I would NEVER do that though!)
I would also call RCCL and see if there was anyway to apply the deposit you paid to a future cruise or if they'll work with you in any way.