Let's make sure that we're on the same page here...are you talking about the Plus *FARE* or the Plus *DRINK PKG*? Princess has really created a TON of confusion with their naming of these two items. I assume you know the difference between the two... (?) I was under the impression that you had to select the *FARE* when you book...which gives you all of its given perks (drink pkg, wifi, gratuities, etc.) If it's the Plus drink package then yes, you should be able to use your OBC once on board to add it.