Yup, as PP poster said it just applies to your balance at the end of your cruise so whatever you received in OBC's will reduce your overall balance at the end of the cruise (everything you do on board/excursions/purchase at Castaway that has a fee is charged to your shipboard account including your gratuities). At the end of the cruise, whatever you had in credits will reduce your overall balance and then the remainder will be charged to whatever credit card you put on file for charging privileges or if you didn't store a credit card you can pay with cash or travelers checks the morning of debarkation at guest services.
Also of note, you should receive a note in your stateroom at the beginning of the cruise letting you know the total credits you have (at least we did in September).