Behind their seemingly picture-perfect lives, the couple struggled to start a family with Deb going through several miscarriages and failed rounds of IVF.
"We did IVF and Deb had a couple of miscarriages. I'll never forget it, the miscarriage thing… It happens to one in three pregnancies. But it's very, very rarely talked about. It's almost secretive," Hugh explained of their heartbreaking fertility journey during an appearance on the Katie Couric talk show in 2012.
"It's a good thing to talk about it. It's more common and it is tough. There's a grieving that you have to go through," he added.
While the couple always wanted to adopt, they fast-tracked their plans after realising they wouldn't be able to conceive naturally.
They initially started to look at adopting within Australia but were forced to move their search to America as a result of complicated local adoption laws.
Hugh, 52, said he doesn't think of Oscar and Ava as adopted, rather as him and Deborah's blood children.
"We feel things happened the way they are meant to. Obviously, biologically wasn't the way we were meant to have children," the Wolverine star told the Herald Sun of his family unit.
"Now, as we go through life together, sure there are challenges, but everyone's in the right place with the right people. It sounds airy-fairy, but it's something we feel very deeply."
After going through the arduous adoption process herself, Deb was inspired to use her platform to raise awareness and become an adoption advocate, campaigning to change international adoption laws to make it easier for loving families to welcome in-need children into their homes.
"I can't think of anything more important than ensuring that every child in this world has at least one person who has their best interests at heart, someone who cares about them so they know they are loved and valued," the actress explained in an impassioned essay for The Australian Women's Weekly in 2011.
"Every child deserves to be educated and nurtured so they can grow in a positive way and be able to eventually become a happy and contributing member of their community."
In 2019, Hugh opened up to People Magazine about navigating the challenges of his kids growing into young adults.
“My son is about to leave high school, and my daughter is about to go into high school. It’s amazing to just have conversations with them. But it took a bit of readjusting,” he said at the time.
“It went from ‘Hey, guys, these are the rules, we don’t do that,’ to having to give them reasons why.”
Despite the changing family dynamic, The Greatest Showman said being a father has taught him more than he ever could have imagined.
"When your focus is innately, deeply, 100 percent [about] these kids in your life and their well-being, it just seems to put everything into perspective," he said.
Hugh also revealed he even once "wing-manned" his son when he was 15 and had a crush on a girl.
The Golden Globe winner said he helped Oscar talk to a girl by revealing he played Wolverine in the popular X-Men movies.
"Once he was on the beach. He was about 13 and he was talking to a girl, who I guess to be about 15, and he started walking towards me,” he previously told Entertainment Tonight.
"He came up to me and he started walking ahead of the girl, and he goes, ‘Dad, Dad, Dad, she’s coming over. I told her you’re Wolverine, just go with it.’ I was like, ‘I am the wingman for my boy.'"
When Oscar turned 18 three years ago, he made the brave decision to reunite with his birth family.
In a caption accompanying an Instagram post of the foursome's first family photo in February 2018, Oscar touchingly penned a tribute to his aunt and sisters.
"Today has undoubtedly been the most important day of my life. I finally got to meet my biological family. Although I was only in Iowa for half a day, I felt that I got to know each of you personally," he wrote.
"You are some of the kindest people I've ever met, and I promise I'm not just saying that."
Olivia responded, "I feel like I've known you my whole life. Love you forever and always little brother."
Her message also appeared to confirm that Hugh, Deborra-Lee and Oscar's younger sister Ava were also there for the special occasion.
"You and your family are very kind and accepting humans that I hope to get to know better," she wrote.
Oscar's mum Amber gave birth to him in California in 2000. She didn't introduce him to her family before he was adopted, nor reveal the infant's birth father.
Amber, 28, died by suicide in 2005, one year after speaking out to an American magazine to claim that the only time she sees Oscar is when he's photographed with Hugh by paparazzi.
Oscar has since found love with a young woman who shares a similar upbringing to him.
His girlfriend Grace Hingston-Hurtado is the daughter of Aussie actor Sean Hingston and his American husband TV producer Brad Hurtado, who are best remembered for playing gay couple David and David, who tried to sleep with Samantha Jones on Sex And The City.
The couple adopted Grace, 20, from San Antonio, Texas, in 2000 – the same year Hugh and Deb adopted baby Oscar from a family in Vinton, Iowa.
It's understood the college-aged kids, who both attend school in New York state, met through mutual friends and have become inseparable.
Grace lives in Brooklyn with her two dads, which is a short jaunt to the West Village penthouse apartment Oscar shares with his parents and his sister Ava, 15.
Oscar seems to be following in the footsteps of his philanthropist mum by becoming an impassioned environmentalist and human rights campaigner.