The suspect and another person were injured and taken to a hospital, police Lt. Jennifer Amat said, adding that there was no ongoing threat to the public.
Police said officers arrived to an active shooting scene about 5:30 pm local time.
There was a shooting involving an officer, police said, but more details were not immediately clear. The officer was not injured, Amat said.
Amat did not know the conditions of the suspect or the other injured person, NBC News reported.
The building appeared to have several offices. Amat did not say where the victims were found, but said of the incident, "as far as we know it was an actively evolving situation, so it was not contained to just one area."
Gov. Gavin Newsom called the events "horrifying and heartbreaking," and said in a statement that "our hearts are with the families impacted by this terrible tragedy tonight."
Orange is a city of about 140,000 people, 30 miles southeast of Los Angeles, California.