California_Dreaming
Discussion of characters, spoilers, and fan fiction. [registration required]
http://s4.invisionfree.com/California_Dreaming

Coffeerooms the OC
General show discussion.
http://www.w3pg.com/forums/tv/oc/

2D1@The O.C. Forums
Discussion of characters and the actors who play them.
http://theocryan.proboards7.com/index.cgi

eFanGuide: The O.C. Forum
General show discussion.
http://efgforum.com/6/ubb.x?a=frm&s=590606&f=75960557

Fan Forum - The O.C.
General show discussion.
http://www.fanforum.com/forumdisplay.php?f=152

Fox Forum
Official forum sponsored by the network.
http://forums.prospero.com/foxoc/

Neurox: The O.C. Forums
Discussion or cast, fashion, rumours,and music.
http://theocboards.com/

The OC @ www.ezboard.com
Show discussion with spoilers, episode guide and fan fiction.
http://p213.ezboard.com/btheoc52538

The OC California
Includes email list, pictures of cast, avatars, quotes, and episode guide.
http://groups.msn.com/TheOCCalifornia/homepage.msn

The OC Forums.com
Message board with topics including the actors, the show, spoilers, and general discussion.
http://www.theocforums.com/

Yahoo! Groups : The_OC
Mailing list for show discussion. [Yahoo! registration required]
http://tv.groups.yahoo.com/group/The_OC/