Here are some of the more popular ones for starters:
BIS OFP Forum
The OFP Editing Center Forum
SIMHQ's OFP Forum - Read Only
Codemasters OFP Forum
The ALT.GAMES.OPERATION-FLASHPOINT Usenet newsgroup (OFP NG). The link many not work, depending on your ISP's news server name, your ISP's news service not supporting subscriptions for the OFP NG, your NG browser's setup or other anomalies.
It is highly recommended that before you post any questions on these forums, that you use each forum's SEARCH facility to look for existing topics, threads and posts that may already provide the answers you are looking for. The forum regulars will be forever indebted to you for this. Some questions have been asked over and over again.
To search through past posts in the OFP NG, I recommend using Google Groups. Under the SEARCH box there, make sure you click on the "Search only in alt.games.operation-flashpoint" radio button before starting your search.