In the lore, the little omnics or the "shooting dogs" appear only on the PvE event. That type of omnics are used only for military things, like Bastion (in the event, Bastions appeared to try to kill you). On the other hand, Zenyatta is like a "human" omnics. They are just person, created to live and help people, not created to kill person, fight war, etc. Orisa (OR15-A) was created to secure people on the airports, police station, etc. But in the past, a rebellion appeared, the rebellion on the omnics, so OR15-A want to kill humans, or bastions, etc.
In short, "Zenyatta's" were created to help people and "Orisa's" for military help.