Thousand Oaks is in Ventura County, about 40 miles from downtown Los Angeles.
Ventura County Sheriff's Office Captain Garo Kuredjian called the situation "active", saying there were at least six people injured inside the building. The extent of the victim's injuries has not yet been released.
He said he heard from people inside that they were hiding in bathrooms and the attic of the bar.
Authorities are still treating it as an active shooter scene.
Authorities say there were multiple injuries - including one officer - after a man opened fire in Southern California bar late Wednesday.
There were unconfirmed reports of multiple shots fired and that the suspect may have been hit, as well a deputy.
Local reports say between 11 to 20 people have been injured. More information will be provided as it becomes available.
Police are responding to reports of a mass shooting at bar in Thousand Oaks, California. Law enforcement officers hovered around the bar with flashlights and weapons were drawn.
Police tell the Los Angeles Times that at least 30 shots were fired.
Witnesses describing the moment the gunman opened fire said the man first shot the guard at the door with a handgun.
Officials said hundreds of people, most of whom college students, were inside the establishment at the time of the shooting, according to KABC.
"Please stay away from area", the department urged in a tweet.