During Tuesday's parliamentary ceremony, the oath was administered as the nation began its official period of mourning. King Harald V's coffin is lying in state at the Royal Palace, where mourners have begun paying their respects. The public viewing is scheduled to continue until 8 September.
Queen Mette-Marit's absence from the formalities was noted; she remains in recovery following a lung transplant. No further details about her condition have been released.
The new king delivered his first speech in a separate event, in which he remembered his late father, according to a related report from the BBC.