I was facing something similar with my recent pregnancy, my OB was on leave for the whole month of October (not as long as yours), and I was due on the 30th, but likely to go earlier.
But she knew me and my history and I wanted to see her for as long as possible, and I knew that even if I did deliver before she was on leave and it was her weekend off (as she alternated with her partner OB) I wouldn't be guarenteed she would be there anyway. In the end she did deliver DD2, and I had her partner OB do my aftercare in hospital.

Is the other OB her partner/backup OB? Or is it someone completely different? Would you be able to meet with the other OB earlier jsut to at least be introduced etc?
Tough decision, because I know how important having an OB you trust and who understands you is
Take care xxoo