As mentioned in this FAQ item, OFP's original smoke grenades do absolutely nothing to cover you in SP missions. And although customized smoke grenade addons are now available that do block OFP AI's line of site, unfortunately players who want to cheat (or to innocently reduce processing overhead) can customize their OFP settings so that smoke won't display for them when playing OFP - both in SP and MP games.
This can be done by disabling the display of cloudlets via the OFP preferences program. When cloudlets are disabled, smoke is not rendered on that player's game display. In this case, players who think they're hidden behind a cloud of smoke might just as well live in glass houses.
From the above and from the general FAQ item on smoke grenades, it can be concluded that the only time when you can be 100% assured of the proper use of smoke grenades against the enemy in MP is when you are playing a coop mission against OFP's AIs and you are using one of the new smoke grenade addons designed to block the AI's line of site.