Perhaps I'm misunderstanding this part...
On our last Celebrity (Reflection B2B) sailing, we had something like $800 OBC for each leg. I was able to draw on that prior to sailing for excursions and such - ended up using most it ahead of time to get dozens of their $55 promo chips for $50.
On our upcoming Mardi Gras sailing, each time I've purchased something for that cruise (Wi-Fi, specialty dining, etc), I'm given the option of using my OBC. I, instead, use the handy $500 AARP Carnival gift cards - which I purchased for $450 each. 🙂
Tom