Yep, anything by BAS are the big must-haves.
If you want to create your own missions you'll want Kegetys Editor Addon.
There are jungle, desert and arctic versions of the official islands - it's well worth looking at one of each, they give you a different perspective on the place.
Upgrade to version 1.91 of OFP from the official BIS site
www.flashpoint1985,com which I think also has a couple of official addons that you can't get else where. The Sopwith Camel at least.
Chain of Command is a script base mod that you should know about. Also the Enhanced Configuration Project - still in beta stage but mostly working pretty well. Having that while you play the campaign will make a difference.
You'll also need Amalfi's pbo decryptor 1.5 if you want to open up missions or addons. Get it from the Editors Depot, along with CoC and ECP links.