For a first cruise with Disney, you can get OBC through a TA only.
While on your
Disney cruise, you can do an 'on board booking'(OBB) and either book a new cruise or a placeholder cruise (to be used on a cruise within 24 mths and you are charged a $250 refundable deposit) that will give you 10% off that next cruise and an OBC of either $100 (under 7 days) or $200 (7 days or more). OBB's are subject to blockout dates--normally around holidays and 2 day sailings. With your OBB, if you list your TA or transfer it to a TA within 30 days you can get OBC from your TA as well. A lot of TA's will offer 1/2 the OBC they give to new bookings due to their reduced commission for these bookings. Costco is one that will give you the same amount back on a Costco gift card after the cruise, I believe people have said it's 8% of the cruise cost?