The Optical Sight is an attachment in Far Cry 3. It is a red-dot scope that increases accuracy and range, as well as providing a wide field of view compared to traditional iron sights. It also has a small magnification (presumably 1.6–2.0x).
It is currently unknown what the exact number of weapons that can use this attachment is, but it appears that it can be used by all assault rifles and submachine guns. Although intended for longer range engagements compared to either iron sights or red dot sights, the optical sight in Far Cry 3 might in fact be detrimental to the players sharp shooting capabilities as the illuminated crosshair in the scope is crudely defined and at longer ranges may in fact hide entire enemies behind it, making headshots near impossible to land reliably.