The man, identified by media as 21-year old Philip Manshaus, is suspected of murder in the death of his 17-year-old stepsister and attempted murder at the al-Noor mosque on Saturday.
But police are considering adding a charge of “attempted act of terror.”
“I can confirm” he rejects the allegations, his lawyer Unni Fries told AFP.
The suspect is due to appear before a judge at one p.m. (1100 GMT) for a detention hearing. Police have called for it to take place behind closed doors.
The suspect entered the mosque in the affluent Oslo suburb of Baerum armed with at least two weapons and opened fire before being overpowered by a 65-year-old man who suffered minor injuries.
Hours after the attack, the body of a young woman was found in a home in Baerum and police on Sunday confirmed that it was the suspect’s 17-year-old stepsister.
According to local media, she was of Chinese origin and had been adopted by the companion of the suspect’s father.
Oslo’s acting chief of the police operation Rune Skjold said Sunday the investigation showed that the suspect appeared to hold “far-right” and “anti-immigrant” views.