Army OCS page - A lot of great information, especially under "Student Information" and "FAQ" - http://www.benning.army.mil/infantry/199th/ocs/
Army OCS Recruiting - brief overview of Army OCS - http://www.goarmy.com/ocs.html
Wikipedia - https://en.wikipedia.org/wiki/Officer_Candidate_School_(United_States_Army)
Army OCS portal - need to sign up to access information, but considered the unofficial Army OCS information source - http://www.armyocs.com/
Army OCS Behind-the-scenes video - check out several other recruiting videos for OCS uploaded by GOARMY - https://www.youtube.com/watch?v=vVhHWCfV5TI
Navy
Naval Officer Training Command - general program information - http://www.ocs.navy.mil/ocs.asp
Wikipedia - https://en.wikipedia.org/wiki/Officer_Candidate_School_(United_States_Navy)
Video Overview of Navy OCS - has several additional videos that address various facets of the OCS program - http://todaysmilitary.com/videos/overview-navy-officer-candidate-school
Navy OCS Summary Video - overall a very comprehensive look into the day-to-day activities of students at OCS - https://www.youtube.com/watch?v=k6LBuyxy6Ns
Survivor's Guide to Navy OCS - a great resource that talks about in detail a day by day then week by week basis on all the various activities at OCS and how to prepare for them - http://www.navydads.com/group/navyocsdads/forum/topics/survivors-guide-to-officer-candidate-school-ocs
Air Force
AF OTS Recruiting - brief overview of AF OTS - http://www.airforce.com/joining-the-air-force/officer-overview/
OTS Homepage - general program information - http://www.au.af.mil/au/holmcenter/OTS/index.asp
Air Froce OTS portal - need to sign up to access information, but considered the unofficial Air Force OTS information source - http://www.airforceots.com/portal/index.php
Air Force OTS Class Video - provides a small snapshot into day-to-day activities of some students of OTS - https://www.youtube.com/watch?v=RtIS7zu46PY
Marine Corps
Marine Officer Recruiting - brief overview of OCS - http://www.marines.com/becoming-a-marine/officer-candidates-school
Wikipedia - https://en.wikipedia.org/wiki/Officer_Candidates_School_(United_States_Marine_Corps)
Marine OCS Training Command - provides some general OCS and candidate information - http://www.trngcmd.marines.mil/Units/Northeast/OfficerCandidatesSchool.aspx
USMC Officer - great resource with a lot of information in nearby links - http://www.usmcofficer.com/officer-candidate-school/
Marine OCS Recruiting Video - https://www.youtube.com/watch?v=gn-CYv5USd8