When should I sign up for DPP?
The short answer is: as soon as you are eligible. This way, if OSU's programs change some of their requirements, you will not have to re-take classes or take different classes from the current program requirements. As a DPP student, you are an OSU student even if you haven't taken a class there, and thus have catalog rights.
At the very latest, however, sign-up about a month prior to the start of the term you'd like to begin taking classes at OSU. Here is the link to the offical DPP application deadlines.