The oculus was a contrivance engineered by the Attendants and put to use by Supreme Leader Snoke of both the First Order and the Sith Eternal. Positioned within Snoke's throne room aboard his Mega-class Star Dreadnought, the Supremacy, its purpose was that of a highly advanced magnifying instrument, allowing the person employing it to more easily observe objects situated kilometers in the distance. In 34 ABY, during the time of the Battle of Oetchi, upon Rey's presentation before Snoke, he compelled her to utilize it to witness the obliteration of Resistance transports as they escaped the Resistance warship, the Raddus, above the world of Crait.
Furthermore, an oculus device existed within the Sith Citadel located on Exegol.