Matt O’Riley made more than 50 appearances for MK Dons during his year at the club
The 21-year-old penned a four-and-a-half year deal with the Scottish giants, who sit second in the Premier League, four points adrift of Rangers.
The former Fulham man spent four days shy of a year at Stadium MK after signing for the club last January. He racked up 52 appearances for the club, scoring nine goals - seven of them this term.
His time at the club was bookended with goals against rivals AFC Wimbledon - scoring at Plough Lane in only his second appearance, while netting the winner at Stadium MK in his penultimate game for Dons.
O’Riley is the fourth player to depart Dons this week, following Ethan Robson (Blackpool), Peter Kioso (Luton Town) and Josh Martin (Norwich City) who all had their loan spells cut short by their parent clubs.
Speaking about the latest addition to his squad, Celtic manager Ange Postecoglou said: “We’re delighted to get Matt into the club. He’s someone who should fit in well with the group we have here, both from on the field and also the kind of person he is.
“We’re looking forward to having him as part of the group. He’s a talented young player and I’m sure he’ll enjoy his time here, and I’m sure he’ll add to what we’ve already started.”