OCS is 17-week course of instruction which prepares graduates of civilian colleges and qualified enlisted personnel to serve effectively as Coast Guard officers. OCS indoctrinates students into a military lifestyle and provides a wide range of highly technical information necessary for performing the duties of a Coast Guard officer. Graduates receive commissions as entry-level Coast Guard officers (ensigns) and are required to serve a minimum of three years of active duty. They may be assigned to a ship, flight training, a staff job, or a shore-based billet. Although individual desires are factors in assignments, they are based on the needs of the Coast Guard. All graduates must be available for worldwide assignment.
Applicants for temporary commissions must be at least 21 and not yet 35 (as of 30 September of the fiscal year in which the selection board convenes).
Applicants for Reserve commissions must be at least 21 and not yet 31 (as of 30 September of the fiscal year in which the selection board convenes).
The other qualifications for OCS aren't listed here because they're quite complex. You can find them on the Coast Guard Recruiting Command's web site.
Information on qualifications for OCS and other non-Academy officer programs is in the Personnel Manual at section 1.B.5.
The application process is involved, but can be completed in a few weeks. The preparation, however, takes much longer and involves a lot of diligence and hard work.