The Outerworld Investigation Agency or Outer World Investigation Agency (abbreviated OIA or, less commonly, OWIA), is a fictional police force in the Mortal Kombat series. The OIA partly supplanted the role of the United States Special Forces in the Mortal Kombat universe after Shao Kahn's failed invasion of Earth and the subsequent destruction of the Black Dragon crime cartel. Though this organization is focused on exploration of other realms, in truth, it is more military in nature.
In the events leading up to Mortal Kombat 3, Sonya and Jax attempted to warn their government about the impending Outworld threat, but their words fell on deaf ears. Shao Kahn's invasion of Earthrealm began uncontested, and he claimed almost every soul on Earth for his own. Jax and Sonya were among a few survivors whose souls were protected, selected by the Elder Gods to fight for their world's freedom. The Earth warriors, led by Liu Kang, were successful.
Jax Briggs' ending in Mortal Kombat 3
The OIA also keeps track of known portals that lead from and to Earthrealm. To open them, it uses "science, rather than magic", as Jax's ending states in Mortal Kombat 3.
Sonya Blade's MK3 ending mentioning the formation of OIA
After Kahn's defeat, Sonya and Jax's superiors were convinced of the potential threat other worlds could bring and gave them clearance to form the Outerworld Investigation Agency (OIA), a subsidiary of the Special Forces, whose job it would be to protect Earthrealm from extradimensional threats. With time, two particular members of interest were added to their ranks: the cyber ninja Cyrax, and the blind swordsman Kenshi who joined in order to make his hunt for Shang Tsung easier.
The storyline of Mortal Kombat: Deadly Alliance suggests that the OIA has special agents on most known realms to identify possible threats to Earthrealm. During the events of Mortal Kombat: Deadly Alliance, the OIA met with catastrophe.Hsu Hao, a Red Dragon spy who had infiltrated the Special Forces, entered the agency's underground bunker and activated miniature nuclear weapons as he escaped, utterly destroying the OIA headquarters, and preventing any further travel to other realms. The Red Dragon felt the OIA to be a threat after its prime purpose had been served: the destruction of the Black Dragon.