Everything that you do/buy on the ship that has a charge is paid for with your OBC first (with the exception of Future Cruise Deposits). Non-fundable is used first, then refundable. If you use all of your OBC, your credit card will be charged.
If you have OBC left over 🤣...<$25 can be cashed out at passenger services. If it's >$25, a check will be mailed (someday). It took about 2 months after our April cruise.
You can also play with your extra OBC in the casino.