His failing body came as a shock to the former marathon runner, hiker and Sydney father-of-two, then in his late 60s.
Concerned, he visited a neurologist who assured him what he was experiencing was just the latest in a string of complications from an old surfing injury.
"Go away and do pilates," was the takeaway, So, much relieved, he took the advice on board.
But over the following 12 months, Mr O'Hehir continued to grow weaker. The problem gnawed at him until he felt his only option was to seek a second opinion.
After one look at his results, the specialist told him: "I'm sorry, it's motor neurone disease. You've got three years. Maybe."
Looking back, it was a "pretty awful Christmas present", Mr O'Hehir told AAP.
Dubbed "the beast" by former Australian of the Year Neale Daniher AO, the disease gradually kills the nerve cells responsible for muscle movement.
Patients gradually lose their ability to walk, talk, swallow and breathe.
About 2800 Australians live with it, while two are diagnosed with it each day and another two die.
There is no known cure and treatment options are limited, underlining the importance of pioneering research and clinical trials.
University of Wollongong's Dr Alexander Mason is spearheading one such effort.
His research team plans to grow miniature, cellular models of the human spinal cord to chart how the disease spreads throughout the body.
The models will mirror the complexity of real spinal cords, allowing researchers to study MND at a level of detail not possible in living patients or via existing research.
"It's really important for understanding the disease to look at the connection between different motor neurons and how the disease spreads between different regions," Dr Mason said.
"It's something we currently can't do.
"If we can arrest that spread or halt it, then you potentially prolong that person's life."
Funded by MND Australia, Dr Mason's work could allow researchers to observe how treatments interact with the disease in microscopic detail.
"The platform can help researchers prioritise the most promising drug targets and therapies earlier," project collaborator Dr Dzung Do-Ha said.
"It's not a direct treatment for people with MND but it's an important step toward more precise and targeted research that could ultimately shorten the time it takes to develop effective therapies."
While the research does not promise a cure and success is not guaranteed but for patients like Mr O'Hehir, it's a welcome sign science is searching for new ways to understand the disease.
"I hold out big hope that research will find an answer because it's such a difficult disease," he said.
It's been six years since he was told he had half as many to live, meaning the 75-year-old continues to defy the odds.
While MND does not yet seem to have spread beyond his arms, fighting it is a daily struggle.
Driving a car, turning a tap, reaching above chest height and carrying a cup of tea or glass of wine are some of the small luxuries Mr O'Hehir can no longer enjoy.
It's a burden he shares with his wife, Gai, who has developed a strained tendon in her hand from taking over the heavy lifting he would otherwise do.
"That's the thing I really hate," Mr O'Hehir said.
"It must wear on her," he said. "I know it does."
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