I haven't made a purchase from the app pre-cruise. The last time I cruised the app didn't have that capability. But I have purchased items from the "gifts and amenities" section of their website previously, and you have to pay for those with a credit card. I would assume purchases from the app are handled the same way and OBC cannot be used.
OBC can be used for excursions you book pre-cruise because they are not actually billed until you are onboard.
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