Filming took place at the London Pub, which describes itself on its website as “Oslo’s biggest gay and lesbian venue”.
“I saw a man arrive with a bag, he took a gun and started shooting,” an NRK journalist said in the channel’s report.
A suspect was apprehended nearby, but it was not immediately clear what the motive for the attack was, according to the Reuters report, citing local police.
Three people were seriously injured, police told reporters at the scene.
Oslo is expected to hold its annual Pride Parade later on Saturday.