The only SP story Campaigns that come with OP are the pirate campaigns. If you do not see campaigns for all six cartels, then something is wrong. The empires only have general conquest campaigns which have no story at all. You can download the EAW campaigns and the Sulu Bonus missions compiled for OP, and there are a couple of SP skirmishes if you look for them. One place to start is here:
Starfleet Universe Links Just go down to the misisons and API section and follow the links. Make sure that any missions you download are for OP or they will crash the game.
Unfortunately, there does not seem to be a great deal of interest in OP missions. There were a ton of missions for SFC I, and quite a few for EAW, but there are not many for OP. I wish that someone could redo the SFC I campaigns and bonus missions for OP. It's a shame that interest in modding the game has overshadowed actually playing it. As a single player with a slow internet connection that makes MP impractical, I find that I spend much more time messing around with the shiplist and strings and kitbashing than I do actually playing the game.
And a script is the misison file, with the extension .scr. To install missions you must copy the *.scr files into the two scripts folders, one in the main OP folder and one in the Assets folder. If you download campaigns, there will aslo be *.mct files which tell the game which missions to offer and when. They will go in the campaigns folders inside the two scripts folders, i.e.:
C:\Program Files\Taldren\Starfleet Command Orion Pirates\Scripts\Campaigns
C:\Program Files\Taldren\Starfleet Command Orion Pirates\Assets\Scripts\Campaigns