Celtic claimed the Glasgow bragging rights as they beat Rangers 2-1 in a game where Alfredo Morelos was sent-off in the first half.
The Bhoys needed a late winner from James Forrest to take all three points after Ryan Kent cancelled out Odsonne Edouard’s opener.
Celtic defended well for most of the game and Kristoffer Ajer was a man mountain at the back.
The 20-year-old won everything in the air and produced two match-winning tackles in the closing stages of the game.
One tackle, in particular, stood out as he denied Joe Worrall from inside the six-yard box with a full-stretch lunge.
Ajer finished the game with blood on his face but it was a warrior-like performance from the youngster to help Celtic to victory.
Despite his young age, the towering central defender has already played well over 50 games for Celtic and he now looks like a regular in the line-up.
The Norway international could be set for an even bigger role next season with Dedryck Boyata set to leave on a free transfer and Filip Benkovic heading back to Leicester City after his loan.
Ajer proved his credentials with an impressive performance in the Old Firm derby and he showed why Celtic can rely on him next season.