From Halopedia, the Halo wiki
The Orion Arm, also referred to as the Orion Spur, is a spiral escalation of a minor spiral arm in the Milky Way galaxy that is approximately 3,500 light-years across and 10,000 light-years in length. It is located near the galactic core, between the Sagittarius Arm and the Perseus Arm. The Orion Arm is most notably home to the territories of both humanity and the Covenant.
Most of the Orion Arm was under the control of the ecumene until the massive Forerunner-Flood war. The Flood absorbed numerous planetary systems in the Orion Arm. Following the activation of the Halo Array in 97,445 BCE, the surviving Forerunners left the Orion Arm as well as the Milky Way galaxy itself.
Formation of the Covenant
As the Covenant continued to grow, Covenant colonies continued to spread across the Orion Arm.
Rise of Humanity
During the Human-Covenant War, Insurrectionists and other independent human forces began creating settlements on planets on the Sagittarius side of the Orion Arm. These settlements were not sanctioned by the UNSC.
On October 28, 2558, Cortana, the leader of the Created sent an ultimatum. After which, she unleashed her Guardian Custodes on the species, civilizations, governments, and organizations on the Orion Arm.
The Orion complex, the center of the Forerunner ecumene and the location of their homeworld Ghibalb, was located in the Orion Arm. The homeworlds and colonies of the former client species of the Covenant are also found in the Orion Arm.