The Orion Main is an old term for the zone that reaches between the Orion Colonies, Rigel and the Sword Worlds. El Nanth marks the customary border of the area on the Eastern Side, Bajor marks the other end, to the West.
In the first half of the 22nd century, the Orions had the run of the area. They had numerous colonies in various stages of development, had conquered several primitive worlds. Using the area as a base, Orions were moving ever farther afield. They had contact with the Romulan Star Empire and the the Klingons, but they were relatively distant. There was time to cut deals, make in-roads and prep the landscape for eventual rivalry with these two.
There was a running low-level conflict with the Klinzai, but the Orions felt like that Klinzai could not make a move without antagonizing their Klingon Masters.
However, two things caught the Orions by surprise. One was the Sword Worlds. This entity coallesced out of nothing to form a potential rival very quickly. The other thing was the Romulan War. The Orions felt that the Romulans and the local worlds would spend much longer grinding away at each other than they did. Out of the war-time alliance, the United Federation of Planets emerged. In the 2170s through the 2190s, suddenly, the Starfleet was everywhere they looked.
Where the Andorians, Vulcans, and Terllarites by themselves were manageable, by pooling resources - their new Federation was a power to be reckoned with. Earth and Alpha Centauri, previously clients of the Vulcans were now armed with real ships and real weapons.
Starfleet had strong opinions about slavery, smuggling, drug running, theft, murder, violence, rape and pillage. All the good stuff. At first Starfleet was slow and dimwitted. But they learned fast. On the other side, Klinzai wanted to conquer the Sword Worlds just as badly as Botchok did. Botchok found itself in the position of helping the Sword Worlds fend off Klinzai fleets
In 2197, the Federation liberated the Planet Tanith from Orion Warlord Padisha Crumalon. Crumalon had a large fleet and tough troops, but he lost. Crumalon's defeat proved the hard way that rallying under an Orion banner and fighting for territory they way every other empire did was a sucker's game. Some worlds fisted up and prepared to resist the Federation, but the writing was on the wall. Botchok sued for peace and was able to negotiate the very favorable Treaty of Orion Neutrality which is in effect even now.
On the other side, in 2201, the Imperial Klingon Fleet appeared in numbers on the Klingon side of the Main. Orion lords quickly swore fealty to the Klingon High Council, or they were exterminated and replaced by a Klingon or Klinzai Lord who would. The Klinzai fleet aggressively patrolled the zone and ruthlessly oppressed raiders, smugglers, mercenaries, adventurers and all the people who made the Main exciting and fun. There was no thought of suckering the Klingons into giving away the store. They had a tendency to renegotiate with disruptor beams and torpedo barrages.
For the last 200 years, the area called the Orion Main has retained its aura of illicit commerce and a dangerous edge.
The Period from 2050 to 2200 is now the subject of romanticized tales of swashbuckling and derrig do, among Orions, Humans, Klinzai and Klingons. The Orion Holonovels "Tales of the Dead Man's Plan" are iconic of the romanticized view of the area - and are popular all over known space.
This area over laps the Sword Worlds and in it's classic phase extended quite a bit beyond, before Starfleet brought law and order on one side and Klingons brought the Rule of the Empire to the other side.