O/I optics device
From Halopedia, the Halo wiki
The O/I (Ocular/Integration) optics device is a helmet attachment worn by UNSC sharpshooters. They are employed by ODST donning the sharpshooter variant of the ODST armor and UNSC Army personnel.
During the Fall of Reach, O/I devices were worn as attachments UNSC Army soldiers' CH252 helmets and the ODST helmets worn by the ODSTs. During the Battle of Mombasa, ODST sniper Kojo Agu employed an O/I module on his helmet.
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