“Shipboard credit is applicable towards onboard products and shore excursions. This includes shore excursions, private vans or cars during the cruise, La Reserve, Beverage Packages and Privée.
Shipboard Credit cannot be used towards spa reservations made directly with Aquamar Spa and Vitality Center. However, you may use your remaining shipboard credit towards a spa reservation when you arrive on board.”
https://oceaniacruises.zendesk.com/hc/en-us/articles/360009638813-What-products-can-be-reserved-using-shipboard-credit