Wing Hansen will make his full tournament debut against Tonga on Saturday evening following a 20-minute cameo in last weekend's 82-8 win over Romania.
Flanker O'Mahony believes the 25-year-old from Canberra is among the best in the world in his position, as well an asset to the group off the field due to his fun-loving nature.
"He's been a breath of fresh air," said O'Mahony. "An incredible character, good person. Straight away we knew he was a top man.
"A character but above it all he's an incredible athlete and one of the world's best wingers at the moment.
"The overriding factor is that he's a good person and he's seamlessly fitted into our squad. He's been great craic - and you need characters like that.
"The beauty of the game of rugby is the different characters you get and we'd be lost without guys like this.
"Tours like this are made for being incredibly serious but the craic that fellas like this bring make it a great place to be."
Hansen was initially a surprise omission from Andy Farrell's matchday 23 for Ireland's pool B opener against Romania in Bordeaux.
But he was a late addition to the bench in place of the injured Robbie Henshaw before coming on for Keith Earls.
Head coach Farrell dismissed any suggestion that Hansen had originally been left out for disciplinary reasons.
Asked by a reporter why the Connacht player was overlooked in the first instance, Farrell interjected: "Was it you who kept on asking the question last week? Where the hell did all that come from?
"Left out of the 23? No. We wanted to give someone else a game, as simple as that.
"Mack's up next (for media), so you can ask him the question as well, 'was he a naughty boy?'. He definitely wasn't."
However, when an attempt was made to ask Hansen about his coach's comments, O'Mahony, who was sitting alongside him, stepped in, saying: "Did Andy not just answer a question about this a second ago?"
Hansen then joked: "We had a five-minute tiff, whatever, and we're fine now, so it's all good!"
The native Australian, who made his Ireland debut in last year's Six Nations after qualifying through his Cork-born mother, is already relishing his first World Cup experience.
"It's an absolute privilege, I didn't know if I'd ever get the opportunity to do so," he said, ahead of the weekend match in Nantes.
"First start, against a good Tongan team, it's really exciting. I was lucky enough to come off the bench last week so that kind of settled the nerves a bit and I can really enjoy this week.
"A lot of my mates are over at the moment, so it's tough getting Snapchats of them smoking vapes and drinking beers at 12 in the morning while I'm trying to prepare for a game.
"But it's good craic and it's actually been nice getting that aspect outside of playing and seeing how much a World Cup means to people."
Hansen attracted attention following the Romania game by stripping to his underwear after swapping shirts with an opponent and giving his shorts to a fan, much to the amusement of the onlooking Farrell.
"I'm not the first person to take their shorts off after a game, I doubt I'll be the last," Hansen said. "I've been told to keep them on this week, I'll try my hardest."
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