When something is billed to your on board account, the celebrity system supposedly first charges it against your non-refundable credit. Many/most/perhaps some of us have both celebrity OBC's and OBC's from our travel agent. In practice, I'm not sure that anyone really knows. The official policy, as I understand it, is that Celebrity OBC's are not useable at the casino.
It is debatable if you can take this type of OBC out at the casino. OBC's from your travel agent are refundable and you can either cash out at guest services or wait for a refund on your cc or cash out at the casino.BUT if you have a credit from Celebrity, it is NON refundable.and you can't cash it out at guest services.