Here a few URL’s where you can download missions, campaigns and all sorts of addons (weapons, vehicles, islands… you name it and it’s there).

http://www.opflashpoint.org/
http://ofp.gamezone.cz/
http://ofp.action-arena.com/

Now remember there are a lot of missions around, and quite a few new campaigns as well. Some of them are absolute junk and/or faulty:in stead of emptying your silenced Bizon SMG, you are suddenly at your Windows desktop again…

A few good downloadable campaigns are:
Samson’s War on Terror
Samson’s It doesn’t take a hero
(both are good fun to play, but Samson is a good friend of the Bible and assumes that computer games about killing people is a good first step to becoming a christian. A bit twisted, but good fun).
Spetsgruppe Alpha 13
Delta Ops
Two excellent special ops campaigns, in the first one you’re Spetsnaz. Tough missions but very nice.

You can find all four (and a few others, which i either didn’t try or have errors) on http://www.opflashpoint.org/maps.php?pt=Campaign . Remember that these campaigns (and also a lot of separate downloadable missions) often need non-standard addons, which you will have to download as well. Usually, a readme file is attached to the campaign download to tell you which separate addons you need.