Marine Corps. M.O.S help?

Recently ive heard that the only open fields and M.O.S's that are open are logistics, truckdriving, and something else I go to boot camp in September and hopefully leave in December, I believe that the fiscal year ends in november this will mean more jobs open right? so I think ill try to not sign any M.O.S contracts right now and stick with an open contract in the hopes that more infantry jobs open up by then. but what I am relly asking is if I sign an open contract and dont pick an M.O.S contract right now will I have any say in what I get out of boot camp?