The qualifications for OCS are listed at www.gocoastguard.com
as well as at the Leadership Development website
. In short, if you have a four year degree from an accredited college, you can apply at any time. If not, you have to meet the grade and educational requirements listed on those web pages before you can apply.
Rates are enlisted occupations. If you are picked up for OCS, you will no longer have a 'rate' and you would probably end up doing something quite different.