The Omnium army are a major antagonistic force in the backstory of Overwatch. They were Omnics produced when the Omnium became hostile towards humanity due to their mistreatment of Omnics.
Due to Humanity's mistreatment of Omnics the Omnium and their god programs became hostile towards humanity and began using prototype designs for combat Omnics and other battle machines that began attacking humans as the armies of every nation from America to Germany began defending their countries and even humanity itself from the hostile machines. Soon the nations came together and founded Overwatch which ultimately ended the war terminating the god programs and stopping the Omnium's war.
However the war had long lasting efforts on the world. More people began outcasting Omnics from their societies and the extremist group Null Sector attacked King's Row, London and despite the leader of London not allowing Overwatch to interfere Commander Jack Morrison send a group in anyway and they successfully stopped Null Sector, however years later Null Sector's leader is contacted by Doomfist and the 2 meet resulting in Null Sector and Talon forging an alliance and later another group known as the Gwi-Shin begins attacking Korea. Null Sector, the Gwi-Shin and other rogue Omnic groups effectively begin a second Omnic Crisis.