Start with the forums. Almost all of them have separate discussion lists for mission editing. Search for what you're looking for before you ask.
Next, there are loads of OFP mission editing sites. Among them are:
Email me if you have any other OFP editing links you think should be added here.
Don't email me if you have questions about mission making or editing. I am a novice mission maker and I'm giving myself more credit than I deserve when I say that.