Obviously my view from personal experience, and all pre transition to JPA/ACOS Manning ....
Option A - If you are posted to the OCU as an instructor then you should be posted in for an entire tour.
Option B - If you are an ab-initio (ie doing the long course / conversion to type) then, even if the OCU is co-located (you don't give the info) with the only front line sqns of that ac type there is no guarantee you will pass, hence you are likely only to get a posting notice for the length of your course.
Option C - If you are a refresher then you should already (hopefully) know the sqn you are going on to - if that is the case speak to your new boss and see if he would be happy to have you posted in to his strength and then detached to do the OCU. The downside is the sqn pick up your T&S.
Obviously this depends on any jobsworths in your way. If you are option A or C then you should have a chance. Option B I suspect is a non-starter. Move into the mess until you pass!