The head of the mosque described the assailant, who was later arrested, as a young white man dressed in black and said he was wearing a helmet and bullet proof vest.
Police were alerted to the shooting at the al-Noor Islamic center in the Oslo suburb of Baerum shortly after 4 pm (1400 GMT).
“There has been a shooting episode inside the mosque,” Oslo police said on Twitter.
Police first reported that the victim had been shot, but later said one person had sustained “minor injuries” and that it was unclear if they were gunshot wounds.
Norway was the scene of one of the worst-ever attacks by a right-wing extremist in July 2011, when 77 people were killed by Anders Behring Breivik.
Police said there was no indication that people other than the “young man” arrested were involved in Saturday’s incident.
“It is a Norwegian young man, with a Norwegian background. He lives in the vicinity,” Oslo police officer Rune Skjold told a press conference on Saturday evening.
Skjold added that the suspect had been known to police before the incident but could not be described as someone with a ” criminal background”.
The police bomb squad was later spotted outside an address linked to the suspect, according to media reports.