I think it depends on the source of the OBC. If it is part of a promotion then it is usually use it or lose it. If it is for $ spent by somebody (pax or TA) you should get it back. Unless you have already made final payment you should see the fuel charge disappear from your final payment.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016
Sail Windjammer: Mandalay 2017
Star Clippers: Royal Clipper 2015