The 74-year-old took over on an interim basis from Brendan Rodgers and then replaced Wilfried Nancy until the end of last season as he turned the club around.
O’Neill led the Hoops to a Premiership and Scottish Cup double in what was an incredible end to the season for the side who looked down and out half way through the campaign.
O’Neill's delighted to put pen to paper
Delighted to be agreeing this new contract with the club, O’Neill said: “It is once again a great privilege for me to continue as Celtic manager. Last season will live long in all our memories and to be part of that success has in a big way whetted the appetite to work again for more days like those and bring our supporters those moments.
“Of course, none of this could have been achieved without the players and staff and I want to take this opportunity to thank them for bringing that level of focus, determination and talent to bear at the conclusion of the season to deliver the league and Cup double.
“I would particularly like to thank the Board for asking me back and giving me this opportunity again.
“We know of course that Celtic can never rest on past success so we are now aiming to drive forward together in the coming season as we aim to deliver again for the club and our fans.
“I thank our fans for their ongoing support, it means everything to us. I look forward to being back at Celtic Park next season when we will come together and we will strive again to bring our fans the success which that support deserves.”
O'Neill claimed his ninth trophy as Celtic manager by beating Dunfermline Athletic 3-1 in the Scottish Cup final and the board will be hoping he can continue his average of 2.56 points per game in the league next season in what will be another intense title defence.
There are no updates on his backroom staff heading into the new campaign but updates should surface over the coming months.