His mother, Elizabeth Rickards said she her son was “fearless”.
“Ever since he went over, we knew something bad was going to happen but off he went,” she said.
“He was doing whatever he could, it’s extraordinary, he ended going up to the front line and bringing people back.”
In an earlier Facebook post, his mother said he trained to be a soldier and “ended up in the war zone when the Ukraine army came under very heavy fire”.
She said it was there her son’s “luck ran out”.
“As you can imagine our family is grieving and he has left a huge hole in our lives. He left two daughters and a son so part of him is still with us,” she wrote earlier.
During his time over there he was awarded a commendation for bravery from the Commander of the Ukraine Army, she said.
O’Neill leaves behind two daughters and a son, as well as his mother and five brothers and sisters.
“We are very, very sad, he was the youngest, our baby,” Rickards said.
“We have a little bit of him left with us.”
Both Prime Minister Anthony Albanese, and the Department of Foreign Affairs and Trade (DFAT) confirmed they are providing consular assistance to O’Neill’s family.
“We send our deepest condolences to the family,” a DFAT spokesperson said.
The department’s travel advice for Ukraine remains at “do not travel”,amid the Russian invasion, which began in February.