MSC tends to hire in patterns. For example they will first publish master promotion opportunities followed a few months later by 1st officer promotion opportunities, then 2nd, 3rd and so on. So as people move up they leave vacancies that need to be filled. This is not an exact science. MSC will hire out of order as needed.
So for those looking to get on as ordinary seaman I recommend you check to see what promotions MSC is working on now. http://www.msc.navy.mil/civmar/promotions/opportunities.htm (Its 1st officers and able seaman this May.) As ordinary seaman follow the able seaman I would say odds are good that they start looking for ordinary seaman in June.
Don’t bet the farm on it, but it’s a good indicator of likely behavior.
Also I would not recommend accepting a different position thinking you will move into the position you want sooner, it usually is not.