OK, here we go. First off, my apologies to all other board members as I am about to now bore you all to sleep. The Fiscal Year (FY) runs from October 1 to September 30. If you go to OCS in October, you will graduate and come up for orders before January which will put you in the first quarter of the FY. I know you will go to the Basic School after OCS but if the USMC does it the same way we did, you will get your orders to your final duty station when you graduate OCS. Graduating OCS near the beginning of the year means there is more money to fund those orders with, and therefore the orders can possibly be more "interesting" and in more "interesting" places. Towards the end of the year opportunities may not be as good as there isn't the money to fund the Permanent Change of Station (PCS) or Temporary Duty Under Instruction (TEMDUINS). However, as in all things personnel, the needs of the service take precedence and there are always exceptions.
Later, this can help in promotions. How many officers being considered for promotion basically depends on the number of vacancies in the next higher grade. This is not so much true for First LT but kicks in for Captain and above. If retention is really good and the need for officers at the next higher grade is down, then the zone of consideration for promotion (Promotion zone) can be shrunk. If that happens then Year Groups, which are based on the FY can be split. If you are in the front of the Year Group, you less likely to be after the split off date and be considered in the next year's group. If retention is down and the need for officers in the next higher is great, then the front end of the Year Group may be included in an earlier group that is being considered for promotion.
Now for caveats. First, all personnel policy is fluid. It can change and change rapidly. Second, I am Navy and although we are closely related (which most Marines want to disavow), there may be some differences. You may have picked up some Army contamination. Third, I have tried to digest something that covers hundreds of printed pages into two paragraphs.
However, all that being said, I sure as Heck would rather go through OCS at Quantico in October than June. I have been there in the summer. It is hotter than Hell and the humidity matches. In October it will be cooling off some and although it gets cold in November and December, it ain't nearly as bad. Also, there are far fewer "beasties" in the woods when it gets cooler.
BTW, have you seen this site?
http://www.marineocs.com/portal/index.phpOnce again, good luck, Marine!