OBC's that are "given" to you as part of a promotion such as NCL's, Wave, promotion usually cannot be used for gratuities or casino. I.E. You can't use it like "cash".
If it is an OBC that has been actually paid for (You gave extra money, in some way, to get: Refund or Bon voyage credit) then you can use it for anything.
These credits will be displayed on your onboard account as "non-refundable credit" (The one you can't use for casino or gratuities.) or "refundable credit" (These are the ones that can be used for anything and if there is credit leftover, it will be either credited to your credit card or you will receive a check at the end of the cruise.