The bar's website says its hosts "College Country Night" every Wednesday.
Authorities said that they responded to a "shots fired" emergency call, and that some officers on arrivals heard shooting.
The gunman shot a security guard standing at the front of the bar before entering the building.
The extent of the victim's injuries has not yet been released.
"We can't confirm the shooter is in custody at this point", Ventura County Sheriff's Office Capt. Garo Kuredjian said at a news conference.
Social media reports claim the suspect shot the bouncer before throwing a smoke grenade and firing more shots.
"Multiple injuries reported", the Ventura County Fire Department tweeted.
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Dozens of people, including one officer, have reportedly been injured in a bar shooting in Thousand Oaks, California.
Unconfirmed reports say the shooter is still at large and that police are still working to secure the scene at the Borderline Bar & Grill.
Multiple people are believed to have been injured in a shooting at a bar packed with college students in southern California, local authorities say.
This story is still developing and will be updated as more information becomes available. Active law enforcement incident.
There has been a mass shooting at a Californian bar, according to United States media reports.
Police said that at least 30 shots had been fired.
Witnesses said a man fired several shots from a handgun before tossing smoke bombs into the crowded bar.