Using OBCs before you sail?

Please excuse the stupid question, but I assumed that if I wanted to pre-book a couple of shore excursions before our cruise sails in July (Sirena, Valletta-Barcelona) then I could use some of our OBCs..? The website didn't give me the option so I just spoke to the helpdesk this morning and was told that is not their policy. You can only use OBCs when you are actually on the ship, so run the risk of the excursions you want being fully booked. Can this be right?

Yes. It isn't a stupid question but as far as I know on board credit has to be spent on board. If it's from your TA there's a bit more flexibility once you're on the ship, but no flexibility beforehand. If it's from O it has to be spent - no refunds if you have any left over.

That is correct. Some people book in advance, cancel once on board and rebook with obc. Of course that only works if your excursion is not sold out