Oculus

Oculus are fast-moving devices used by the Collectors to protect the Collector base. It is unknown if they are actual fighters piloted by Collectors, or some form of remotely-controlled or automated drone.

Oculus possess powerful beam weapons, similar to other Collector weaponry, and can invade an enemy ship by cutting into its hull after jettisoning part of its armor.

Mass Effect 2
The Normandy SR-2 was attacked by several Oculus when attempting to reach the Collector base. The Oculus were apparently left on guard in the local debris field and only activated as the Normandy approached. While most of the Oculus were destroyed by the Normandy's weapons or by accidentally flying into debris, one was able to penetrate the ship's hanger deck, losing its outer shell in the process. It was eventually destroyed by Commander Shepard.

It is likely that the Collectors gained the Oculus technology from the Reapers.

Jack will be killed by an Oculus' particle beam unless Shepard upgrades the Normandy SR-2 with Heavy Ship Armor.

Capabilities
Offensive

Oculus' beam weapon is much stronger than an ordinary particle beam, and will blast away any shields or barriers in a moment, especially on higher difficulties. Oculus fires its weapon in very long bursts, thus anything longer than a brief exposure to this weapon could be fatal.

Defensive

Oculus has an incredibly strong armor, which acts as its health.

Tactics

 * There are two occasions when Shepard and the team face an Oculus. In either case, it will slowly advance and attempt to take a clear shot with its particle beam. While using Tactical Cloak as an Infiltrator does not stop the Oculus from its relentless pursuit of the player, it will still provide the damage bonus it always provides when you fire.
 * In a first case, you only need to lower its armor to about 50% of its "health", and it will retreat. After the cutscene, an Oculus will invade the ship again, and you'll have to fight it to the death. Explosive heavy weapons (such as the ML-77 Missile Launcher or M-100 Grenade Launcher) are quite effective, however, using M-920 Cain is not recommended as its explosion can easily kill you. The Collector Particle Beam is effective as well, however, you are highly exposed to the Oculus' own particle beam, which is not recommended on higher difficulties. Such powers as Incinerate, Warp, Reave and/or Incendiary Ammo are very good in stripping away its armor. Most biotic powers (except for Warp and Reave) are ineffective. Taking cover is also vital, as well as changing position if exposed to fire. Standard weapons with high armor damage bonuses (such as the M-98 Widow, M-92 Mantis, M-5 Phalanx or M-6 Carnifex) are effective, if a little time consuming and dangerous. Shooting the red "eye" will cause more damage than shooting the silver shell.
 * The Oculus is unaffected by Stasis.
 * The three "shield" powers (Geth Shield Boost, Fortification, and Barrier) are immensely helpful in protecting yourself while fighting the Oculus. Wait until the Oculus is just about to clear your cover, activate the power, and sprint to the other side of the room. If timed correctly, the extra shielding from a maxed-out boost can protect you out in the open all the way to the other side, and the power will be available to you the next time the Oculus reaches your position.

Trivia

 * Oculus is Latin for 'eye'.
 * True to their name, Oculus can see through both the Normandy SR-2's stealth systems and an Infiltrator's Tactical Cloak.
 * If Miranda is brought to fight the Oculus, she'll still be heard calling you on the intercom after the fight is over.
 * Oculus-like drones appear in Mass Effect 3's demo . While they are not necessarily Oculuses, their extreme similiarity to the latter might mean that they have been created by the Reapers, and not by Collectors.