The 18-year-old was studying to become a maths teacher, had a part-time job in a supermarket, was popular with fellow students and friends and had a full, rich, and happy life.
But four months later, in mid-January, his mother Donna took a phone call which was "like a bomb ripping through my life", she says: David was missing. He vanished after a night out with friends.
Eyewitnesses describe him as getting very drunk, very quickly and being almost incapable of looking after himself. Sadly, he could have been almost any young man in any town in Scotland, and the case has struck a chord with the public.
Thanks to a combination of his vulnerability, his age and his family's repeated appeals for help in locating him, David's disappearance has now become one of Scotland's highest-profile missing persons cases. But it seems while the police, public and his father cling on to the belief that he will be found alive, his mother has given up hope her only child is safe and well.
Sitting in the home where she raised her son, Donna O'Halloran, 42, says: "I knew, from the moment that I got that first phone call to tell me that David was missing, that I had lost my one and only.
"It feels like my heart has been ripped out. Common sense, intelligence and the circumstances all indicate the worst possible outcome. I've never felt like my son was missing: I always knew he was dead."
David, from Kilmaurs, Ayrshire, disappeared on Friday, January 18 after an evening out in Stirling, where he was studying to become a secondary school teacher.
He had only recently returned to the city after spending Christmas back home in Kilmaurs with his family.
"Each day is an eternity," Donna said. "I'm barely sleeping and I spent the first weeks sobbing my heart out. I was inconsolable.
"The only thing stopping me from joining David is the support I've been getting from friends, family, my partner Malky, and complete strangers, who want to help.
"This whole thing has been soul destroying. I know now there is nothing I can do for David, but I need to find him and bring him home.
"I need to be able to say goodbye. I have to keep believing we will find him one day. If I don't, I don't know what we'll do.
"David was such a happy boy. His university halls were a home from home for him. He had passed all his exams and everything was going well for him.
"Everyone who knew David, loved him. He was a wonderful, caring young man. It's so tragic, he had so much going for him, it's just such a waste of a beautiful young life."
On the night he disappeared, David is believed to have drunk half a bottle of vodka in less than an hour, before going clubbing with friends in Stirling.
He was asked to leave the Dusk nightclub shortly before 1am, after staff decided he had had too much to drink.
One friend helped him into a taxi and instructed the driver to take him to the university's Murray Hall residences.
But around two-thirds of a mile away from the campus David paid the driver and told him that he would walk the rest of the way. He is then thought to have gone down an embankment and walked through a field until he reached Graham's Dairies.
Nightshift employees, who said David was now wet, pointed him toward the university entrance.
He was last seen, in CCTV footage, at around 2.20am, crossing Henderson Street in nearby Bridge of Allan.
More than five weeks later, Donna is now working with a psychic to help find her son.
Paula O'Brien, who describes herself online as "the people's angel", volunteered her services after hearing about the case.
The pair met by the Allan Water in Stirling to try to locate David and have subsequently taken a boat onto the River Forth.
"Paula came forward voluntarily after one of the family had informed her. She has not charged a penny and has cancelled days and days of clients," Donna explained.
"She has told me she's made contact with David, so I know he is no longer with us. He wants to be found."
Donna also revealed her heartache at clearing out David's room at the university.
She said: "I knew it had to be done, because I knew he wasn't coming back. It was devastating."
David's disappearance has had a heavy impact on the student community in Stirling.
Sam Gibbs, president of the university's student union, said: "It is now over a month since David O'Halloran went missing after a night out in Stirling and the whole university community is deeply saddened that he has not yet been found. It has been a difficult and distressing time and, in particular, his friends and those with whom he worked closely on a daily basis – including his course-mates and tutors – miss him tremendously.
"The university community is trying to take things one day at a time and work with Central Scotland Police to assist them with the on-going investigation.
"Students have been able to access counselling and welfare support at the university which has helped greatly during this difficult time."
Police have drafted in helicopters, mountain rescue teams and trained dogs to help search for David. When he went missing he was wearing only a T-shirt with a black and green checked shirt on top.
Superintendent Gordon Dawson, who is leading the investigation, said: "David was not clothed for the cold weather and had consumed alcohol, which is not a good combination.
"This is a missing person enquiry and my job is to find David. We are – and will continue – doing everything we can to find him."
David's father says he is "100% sure" his son is alive and stressed they were "looking for a person and not a body".
Fighting back tears, Alan McInnes said his family was "incomplete" without David.
He said: "David has been missing for a month and every day we hope that he's found safe.
"We remain 100% sure he is alive and hope somebody has seen him or knows where he is. He may have injured himself and someone has helped him.
"Too many people have said he is no longer with us but we have to remain focused that he is alive and that for whatever reason he can't come home.
"We're looking for a person and not a body. We will continue to believe this until told otherwise by police."
His father made a direct plea to David to come home and complete the family.
He added: "David, son, if you can see or hear this, please know that we miss you, we love you and we all need you home. Our family is incomplete without you."
Although he had only been at university for a few months, David was already popular with a large circle of friends.
His step-father, Malky McDougall, 46, said: "David was a very active lad and like any teenage boy. He was neither an introvert or an extrovert.
"He loved his tennis, both playing and watching. I have so many happy memories of David and they will stay with me forever."
Donna and Malky plan to hold an event celebrating David's life in the coming months.
"With or without David's body, this is something we need to do", Donna said. "People need closure. We need closure."
His mother has decided to speak out because of the large amount of alcohol David is thought to have drunk before he disappeared.
She hopes his story may make other young people drink more responsibly.
"Young people need to be more aware of what they are drinking and the amount of alcohol they can handle.
"It is small comfort that something good can come from all this misery.
"People are already listening to these warnings and David is already saving lives. I'm so proud of my son, and I always will be."
Anyone with any information about David O'Halloran's disappearance is urged to contact Central Scotland Police on 01786 456000