O'Connell was born on 1 August 1990 into a working-class family in Alvaston, Derbyshire. His father, Johnny Patrick O'Connell, was an Irish citizen from Ballyheigue who worked on the British railways for Bombardier until his death from pancreatic cancer in 2009. His mother, Alison (née Gutteridge), who is English, was employed by the airline British Midland before taking on management of her son's career. His younger sister, Megan, is an actress. As the grandson of Ken Gutteridge, a player and later manager at Burton Albion FC, O'Connell aspired to become a professional footballer. He played as a striker for Alvaston Rangers and was later scouted by Derby County FC, where he had trials. After a series of injuries ended his career, he wanted to join the British Army, believing it to be his only realistic option to make an honest living. His parents sent him to the Army Cadet Force when he was 12 with the aim of teaching him discipline, but his juvenile criminal record prevented him from enlisting in the army.
As a youth, O'Connell was in and out of court on charges related to alcohol and violence, and he received a one-year young offender's referral order when he was 17. Regarding his past transgressions, he has described himself as "a product of [his] environment". At age 16, O'Connell left Saint Benedict Catholic School with two GCSEs in drama and English. He later reflected on his "brutal" experience at Saint Benedict: "What I learnt aside from anything academic at school was probably very valuable lessons in terms of how to lie, how to play the game, how to play authority against itself." He took an interest in acting during the compulsory drama classes, and from age 13 he attended the free Television Workshop in Nottingham, where he trained in drama twice a week. He began attending auditions in London, where he sometimes slept outside because he was unable to afford a hotel. He eventually moved to Hounslow in London, working in between acting parts as a farmhand in Cobham, Surrey.
