By carefully reading the question, we are able to discern from its wording the fact that Oasis of the Seas is not a submarine. Instead, wherever she sails, she is expected to float on the surface of the briny deep.
A floating object displaces water equal to its own weight.
So the Oasis must displace
(approx 220,000,000 lbs)/(65 lbs/ft³) =
approx 3,384,615 ft³ of sea water