The Overwatch Transhuman Arm, or OTA (as it is commonly abbreviated to) is the military backbone of the Combine. They are infrequently seen within city limits, but in the Outlands and in some parts of the Canals, they are very common. Overwatch Soldiers (colloquially referred to as Combine Soldiers) are transhuman infantry units, heavily augmented and conditioned into post-human killing machines.
OTA units' ranks vary depending on servers, but equipment tends to remain the same; standard, the so-called ECHO soldiers receive an MP7 Submachine gun, BLADE Division receive AR2. MACE is a shotgun-division that receive Shotgun/Spas-12 and Combine Elites recieve the AR2 and heavy, white-colored armor.
As an OTA unit, most servers will give you 150 health and 150 armor to start. This makes you difficult to kill; yet remember that you are still vulnerable alone. As a result, it is best to coordinate with your allies and use tactics such as flanking and suppression fire to defeat enemies in S2K or S2RP engagements.
Most of the time, OTA units will remain in stasis in the Cities (with many servers offering punishment if you use the character without permission); only during a time of distress will they emerge to perform a mission, then usually return to stasis. This is opposed in the Outlands; they will remain on duty for extended periods of time, usually guarding the base.