Rather than muster a creative excuse, he happily admits he was out last night, celebrating his beloved Derby County win a match.
The 23-year-old can currently be seen in little more than a loin cloth and "too much spray tan" in the stylised action movie 300: Rise Of An Empire.
"I had to get in shape, but I'm always up for bettering myself," says the actor, who's receiving rave reviews for his portrayal of a young offender prematurely moved to an adult jail for being too violent, in the uncompromising movie Starred Up.
Unless you're of the Skins generation, chances are you're not familiar with O'Connell, but this Derby lad's been quietly establishing himself as a leading young talent since he debuted as the self-destructive Cook in E4's teen drama back in 2009.
His success lies in his charismatic, seemingly effortless performances.
Following a global search, Angelina Jolie cast him as the lead in her new film Unbroken, which chronicles the life of Louis Zamperini, an Olympic runner who was a Japanese PoW during World War Two.
Given her fearless reputation, it's unsurprising Jolie was hands-on. "There were occasions where she was testing out the stunts I was going to be doing before me. I was like, 'Be careful Angie!'" O'Connell recalls.
He wasn't star struck in her presence - not through early onset jadedness, rather a desire to get on with the task at hand.
"I find focusing on that more beneficial," he explains. "There's a level of professionalism that I've got to up keep if my working relationships are going to be successful."oo excited, I feel really humbled."
UNBROKEN will be released in December and it's already being touted as a strong contender for next year's awards season.
"I'm just thankful this has happened at a time when I'm mature enough to handle it wisely and with sensibility," he says, oblivious to how this might sound given his age.
But O'Connell's something of an old soul. There's a contemplative streak that's been belied by the reputation he's forged in playing troubled characters - Cook, Pukey Nicholls in This Is England and ring leader Brett in horror thriller Eden Lake among them.
O'Connell agrees. "I usually find myself playing characters who are cocksure."
That includes Eric, his character in Starred Up, who seems destined for a life behind bars.
"I don't consider it an absolute must that I approve of every character I play. It's just best to understand them and I find Eric very easy to understand. It wasn't that extreme a leap in terms of that whole demeanour. It runs parallel with the kind of figures I grew up with."
He doesn't describe his upbringing as "deprived" but there wasn't a lot of money.
"My mum and dad worked hard for a living so I benefited from that, whereas perhaps a few people from the same street were surviving off the state," he says.
His father was employed by the railways, and the free train travel meant O'Connell could make auditions. From a young age, he had a pragmatic approach.
"I'd made the decision to join the army by the time I was 16, if acting didn't happen," he says.
The idea was "totally against" his mum's wishes, so she was relieved when the acting took off.
His first professional job was in the soap Doctors in 2005, followed by a short recurring role in The Bill. This Is England and Eden Lake aside, the next three years included pop-up appearances Waterloo Road and Holby City before he achieved greater recognition in Skins.
"To me, the only obvious route, or only plausible one, was to work hard enough over here," he says.
"I knew I wasn't going to get there [Hollywood] by signing myself into a soap for a certain amount of years and prioritising finance. I thought, as long as I keep the roles genuine, then something's going to give."
He regards 2011's United, a BBC2 film about Manchester United's Munich air disaster in which he played a young Bobby Charlton as a turning point.
"I'm so precious about United because it came at a perfect time, and gave me an opportunity to demonstrate other wisdom."
HE'S JUST bought his own place in London so he can "isolate" himself and focus on work.
"I guess we're trying to facilitate a bit more of an under-the-radar lifestyle than the one I was leading," says the actor, who hit the headlines in 2012 when he dated singer and former X Factor judge Tulisa.
He's single at the moment but isn't looking for a starlet to walk the red carpets with.
"I can never base decisions on anything like that," he says. "That means I certainly won't be taking advantage of my celebrity status to woo women. I've got ethics, me. I'm a bit old fashioned like that."
Now, with Unbroken and the thriller '71 (about The Troubles) still due for release, O'Connell wants to sit back and "see what kind of noise" they generate.
"I'm going to weigh things up and make sure I make the right decision," he says. "I wouldn't like to do anything too predictable."