The head of the mosque described the assailant, who was later arrested by police, as white and said he was wearing a helmet and a uniform.
Police were alerted to the shooting at the Al Noor Islamic centre in the Oslo suburb of Baerum shortly after 4pm.
“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 rightwing extremist in July 2011, when 77 people were killed by Anders Behring Breivik.
Police said there was no indication that more people than the “young man” arrested were involved in the incident, but that they had no further information about the suspect.
“One of our members has been shot by a white man with a helmet and uniform,” Irfan Mushtaq, head of the mosque, told local newspaper Budstikka.
Mushtaq told another Norwegian newspaper, VG, that the man had carried multiple weapons, but that he had been subdued by a member of the mosque.
Mushtaq said he had arrived at the scene shortly after being alerted about the gunman, and had gone to the back of the building while waiting for police to arrive.
“Then I see that there are cartridges scattered and blood on the carpets, and I see one of our members is sitting on the perpetrator, covered in blood,” Mushtaq told VG.
He said the man who apparently overpowered the shooter was 75 years old and had been reading the Quran after a prayer session.
According to Mushtaq, the mosque had not received any threats ahead of the shooting.
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