"Devastated by the Sith centuries ago, the planet Ossus has largely been forgotten by the greater galaxy. Enshrouded in the nebular material of the Cron Drift, the planet is difficult to even reach, with those who survive the journey discovering only hazardous wastelands and abandoned ruins on its surface.
Though it was once a notable center of Jedi culture and learning, the planet has rarely been visited by members of the Jedi Order in recent decades, as Ossus is located well within the Sith Empire's sphere of influence. For their part, the Sith have shown no interest whatsoever in exploring the planet; what purpose could be served by visiting the charred grave of an already-defeated foe?" -- Ossus, codex entry.
When the Jedi Order expanded there from the planet Tython and established a Jedi Academy, Ossus would become an important world in the Galactic Republic. The massive data amounts and artifacts there serve as key reminders of Jedi culture and it also serves as a place for Jedi Masters to meditate away from outside influences.