O'Neill was formally sworn in at 3:56pm by Governor-General Sir Bob Dadae.
This follows his election, 60-46, by parliament moments earlier.
Present to witness was New Ireland Governor Sir Julius Chan, Western Highlands Governor, Paias Wingti, and several returning and new MPs.
Also present was his wife, Lynda Babao-O'Neill, who was emotional and embraced her husband before formalities.
A brief toast to end the formalities was followed by a chat with guests before Sir Dadae departed.
O'Neill is on his way to Parliament.