[16] In January 2012, Wallin defeated Frazer Clarke 32:11 in a Sweden–England dual match in Uppsala,[17] which qualified him for the 2012 European Boxing Olympic Qualification Tournament.

[18] Wallin won bronze at the Gee-Bee Tournament in March, losing to Magomed Omarov in the semi-finals.

[19] At the European Boxing Olympic Qualification Tournament in April in Trabzon, Turkey, he was eliminated in the round of 16 by Mihai Nistor.

He trained under Torsten Schmitz for six months in Berlin, Germany, then moved to Copenhagen, Denmark, where he met former two-weight WBA world champion Joey Gamache, who became his trainer.

[23][5] On 14 September 2019, Wallin faced the former unified heavyweight world champion Tyson Fury (28–0–1, 20 KOs) at the T-Mobile Arena in Las Vegas.

The examination by the doctor seemed to motivate Fury, as he poured on the pressure afterwards, hurting Wallin repeatedly with solid shots.

Despite fatiguing in the second half of the fight, Wallin came back in the twelfth and landed a strong left hand which seemed to trouble Fury.

[27] On 26 February, Wallin was removed from the card due to a foot injury and was replaced by Apti Davtaev.

[29] Wallin's next fight was against former IBF and WBC title challenger Dominic Breazeale (20–2–0, 18 KOs) on the undercard of Adrien Broner vs. Jovanie Santiago on 20 February 2021.

The Swede was in control the whole fight and regularly found the target with his straight left hand, causing swelling around Breazeale's right eye.

[30] On 15 September 2021, it was reported that terms had been agreed between Wallin and the WBC interim heavyweight champion, Dillian Whyte (28–2, 19 KOs), for a bout on 30 October at the O2 Arena in London, England.

[31] However, ten days before the fight was scheduled to occur, Whyte withdrew due to reportedly suffering a shoulder injury.

[39] On 15 November 2023, it was announced that Wallin would face former unified WBA, IBF, WBO heavyweight champion Anthony Joshua (26–3, 23 KOs) on 23 December 2023.

[40] Wallin found himself perpetually on the defensive, unable to penetrate Joshua's formidable jab and right hand, which effectively neutralized his offensive capabilities.

[41] Before the start of round 6, Wallin's coach Gamache made the decision to stop the fight, resulting in a corner retirement win for Joshua.

[42] On 27 November 2024, Wallin was announced as a step in to fight Derek Chisora (35-13, 23 KOs) at Co-op Live in Manchester.

[46] Wallin received a call from his manager Jolene Mizzone, the day after his birthday, to offer him the fight.

Chisora tired himself out in round 8, when he threw a 30-punch combination, however not all clean punches and not all landed, but Wallin was hurt from this.

Wallin stumbled back a few stops and lost his footing, ending up staggering backwards across the ring, onto the canvas.

Wallin said his father, an amateur boxer and part-time trainer, motivated him to get into boxing, and hoped to use his father's death as extra motivation to upset the odds and defeat Fury,[54][56] similar to Buster Douglas's losing his mother before fighting Mike Tyson.

Fury said in his in-ring interview, "Big congratulations to Otto and I just wanna say rest in peace to his father.