Before the collapse Osiris was a man of science. He once worked closely with the Ishtar Collective, studying the Vex. However due to several disagreements he thought it would be better if he left and forged his own path. Osiris died during the Collapse.

Osiris was Risen in the Dark Age of Earth by his ghost, Sagira. He spent much of his time alone, scraping by to simply survive. Eventually he would come to patrol with the Iron Lords, and was personally trained by Felwinter and Nirwen, though he never became an Iron Lord himself. Felwinter, unable to provide anymore answers for Osiris’s curiosity, pointed him in the direction of the early City where he would first meet Saint-14.

After the battle of Six Fronts  Osiris would meet the Speaker. Saint was sent off to bring the fight to the Fallen which would leave the Vanguard Commander position open. Saint suggested that Osiris should become the commander, which he readily accepted. Eventually Osiris would grow unhappy with running strikes and holding sessions with Cryptarchs that lead nowhere. The Speaker took notice and took him under his wing trying to help him find his path.

At some point Osiris took Ikora Rey as a student.

Osiris’s questions and insatiable curiosity grew to be too much to handle. He would eventually nearly lead the City into ruin by ignoring the fact these questions caught momentum and created a rift. 

From Osiris and the Speaker’s clashing he would eventually be exiled from the City. Although he would claim to leave willingly.

During his exile, Osiris left for Mercury where he would enter the Infinite Forest and spend centuries, perhaps even millennia, fighting the Vex and simulating different realities and futures in a bid to understand the Darkness. To fight the Darkness.

In this time he would eventually make contact with Eris Morn to speak with Mara Sov about Crota and Oryx.

Sometime after the Red War, Osiris would learn of a Vex Apocalypse and send Sagira to safety while he looked into it more. He would receive help from  a Guardian in defeating Panoptes.

Following this, Osiris created the Sundial to bring Saint-14 back after discovering his tomb. 

Current events.

The Traveler is without voice. Once, the Speaker held that place, received the nightmares the Traveler mourned over. But its one mouthpiece is now dead. So it reaches out, and what it finds it clings to. A light, brighter than others,  even with the potential for a consuming darkness. It starts like this: a dream, a vision, symbolism he’s unsure of, screams and pain and darkness. Osiris woke, a headache pounding his skull. The Traveler chose him once, through Sagira, and it chose him again.