The forums are all full of existing threads with spoilers for practically all of the campaign missions. Just search the forums for the name of the campaign mission you're interested in.
Gamespot has a very comprehensive OFP game guide, with a walk-through and much more. Check out the free downloadable game guide, in PDF format, while you're there.
Of course, if you've bought OFP Gold or the OFP Gold Upgrade, you'll have a copy of Prima's Official Strategy Guide, by Michael Night.