I can't speak for your dive centre, but if you're doing your OW course in a drysuit anyway, you will already be covering your pool drysuit orientation and most of the core drysuit skills within the OW dives. This may mean you only need to complete knowledge reviews and one additional dive for drysuit certification. It's not uncommon for drysuit specialty to be offered at a significant discount if combined with OW, but again, check with your centre.