The Order? I thought they were effectively removed from the game once you finished the campaigns?
Omicron Minor and Major were parts of the SP campaign. On an unmodded game, there is no way I know of for getting back into them once the campaign is over.
The two 'Unknown' systems are very much accessible. I shan't say where even thoug this is the spoiler forum, but there are jump holes. You just have to find 'em
I must have died a hundred times trying to find a way through that minefield towards Alaska. It's a system you enter during the SP campaign, and I think it's impossible to get back into afterwards. Fly all the way around it and check for yourself if you like -- you can't get in without a modification of some sort. The mines must have drifted over the entrance after all that action or something. But I reckon you see everything there is to see in the campaign, so you're probably not missing too much.
"If it aint on fire, you haven't shot it up bad enough yet" - my old teacher on Planet Housten.