Non refundable obc

:confused:Is it stated anywhere on ncl that non refundable OBC can NOT be used for Gratitudes/dsc?

It's in the Terms and Conditions of whatever given promo includes the OBC. Usually says something like:

Onboard credit has no monetary value and is non-transferable. It may not be used toward onboard service charges or prepurchased activities.

https://www.ncl.com/termsandconditions/promotions?hide_hf=true

Edit to add that I believe non-refundable OBC can be used to purchase gratuity vouchers which may be given to your steward or butler, for example. I have not done this personally. But do note that this is not the same as the discretionary service charge to which I am sure you are referring.