"We both agreed it seemed such a tragedy and waste of life for a man who had fought in France, was wounded three times and received the Victoria Cross to spend 12 years in various asylums and mental hospitals," he said.
"It was due to post-traumatic stress, which wasn't diagnosed until the Vietnam War. O'Meara was badly treated because they really were not equipped in those days to treat the mental horrors of war.
"The attitude was 'if a little medication helps, then a little more must help even more' – but it caused quite a lot of damage.
"I thought it best to allow the audience to imagine what it must have been like instead of the in-your-face approach theatres are filled with these days."
"I don't think I could truly capture what O'Meara went through during his army service and his time in the asylum," O'Neill said. "That all came from imagination, thinking what it must have been like.
"But I must say, during previous performances, the veteran soldiers were deeply moved so I felt I had touched something within them."
Under Any Old Gum Tree plays at 7pm May 1 and 2 with a 2pm matinee May 3. Tickets are $25, $20 concession, $10 children, $60 family – book at www.trybooking.com/HEGN.