"I met Clint Eastwood and he told me he liked my film," the actor says.
"I am still not convinced he had any idea who I was, and he was just playing it cool, but nonetheless there was a fleeting moment where I took him at his word."
The Irish star of Bridesmaids also met Tinseltown's golden couple Brad Pitt and Angelina Jolie (who confessed to having watched the hit comedy four times), and has been given the royal seal of approval by the Duke and Duchess of Cambridge.
"I met William and Kate recently and they had watched Bridesmaids a week after they married," O'Dowd reveals.
There is even photographic evidence, which takes pride of place at his parents' home.
"There is a picture that somebody gave me at my wedding of myself and Dawn (O'Dowd married TV presenter Dawn Porter in August) talking to Kate. I had clearly told a big joke they are laughing at. It's a good one," he continues.
"It's right as you walk in through the door. Kate's blown up to 8 x 10ft. It seems excessive!"
As well as being amusing anecdotes, O'Dowd's A-list encounters are evidence of his recent rise to stardom.
The 33-year-old became a familiar face on TV playing computer geek Roy Trenneman in Channel 4's The IT Crowd, which ran from 2006.
With appearances in Dinner For Schmucks and Gulliver's Travels, O'Dowd began to garner a following in America, too, but it was his portrayal of dashing cop Officer Nathan Rhodes in 2011's Box Office hit Bridesmaids that really got him noticed.
"It has definitely opened doors for me. Obviously, it is very helpful, because most people would not have seen me otherwise," he says.
Sligo-born O'Dowd admits that now being called a 'sex symbol' is taking some getting used to.
"I wear that status like a hair vest. It's very fleeting and it's been fun, but I can't be dealing with all that nonsense, because it's not real," he quips.
It's thanks to Bridesmaids that he was cast as band manager Dave Lovelace in The Sapphires.
It is based on the stage production of the same name and the musical drama, set in 1968, traces the journey of Aboriginal girl group The Sapphires, played by Jessica Mauboy, Miranda Tapsell, Deborah Mailman and Shari Sebbens.
"I like to think I was the first choice, after the first six people turned it down. But I am big in the indigenous communities, you know? I like the idea I have a very niche market," jokes O'Dowd.
The movie received a 10-minute standing ovation when it premiered at Cannes, has been a resounding success in Australia (it was the highest earning Australian film on its opening weekend) and is set for success in America after distribution rights were snapped up.
"I think it tells a reasonably unknown story to a wide audience, and I'm proud we managed to create a film where people who have been oppressed are celebrated," he says. "These are joyous, sexy, strong, sassy and beautiful women, and these stories are not told as often as they should." .
As Lovelace, the actor enjoyed belting out soul tracks such as The Temptations' I Can't Help Myself (Sugar Pie, Honey Bunch).
"I'm big into soul music and coincidentally at the time, I had been listening to a lot of soul music and gospel music – a lot of early Sam Cooke, Al Green and that sort of stuff," he says.
While he didn't need singing lessons, O'Dowd did have to learn to play the piano.
"I didn't have a clue how to do it beforehand, so I had to learn it pretty quickly. And far from the romance of the ballrooms in the 1960s, I was teaching myself while I was working on another job at the time," he recalls.
"So I was flying from Sydney to Los Angeles and teaching myself how to play the piano on an iPad app, which didn't really feel in keeping with the beautiful nature of soul music."
With four females on set – Mauboy is a former Australian Idol finalist – the set was filled with song. Having grown up with three older sisters, though, O'Dowd didn't mind being outnumbered.
"Four beautiful women is my go-to place. I grew up with a house full of women so I felt very comfortable," he admits.
"They were like a little family, and I felt at different times like their younger brother, older brother, their uncle and their drunk cousin. But I enjoyed them all. They're just the loveliest women."
Despite his film success, O'Dowd hasn't forgotten his TV roots and has a number of small screen projects on the go.
He recently starred in BBC series The Crimson Petal And The White, has a recurring role in new much-talked-about US sitcom Girls, and created and wrote Sky TV series Moone Boy, which is due to return for a second series. Soon he will be penning and producing a new American comedy series called Big Men.
His upcoming big-screen outings see him tussling with Megan Fox in Judd Apatow's Knocked Up sequel, This Is 40, taking on Nick Frost on the dance floor in Cuban Fury and returning to his roots in Irish comedy drama Calvary, opposite Brendan Gleeson.
In his own words, he's "making hay while the sun shines".
l The Sapphires opens in cinemas on Wednesday