Yesterday, O'Mahoney was given eight years behind bars for her part in five random attacks on eight innocent victims.
O'Mahoney was part of a Clockwork Orange-style gang of youths who took "pleasure" from the infliction of pain.
One of the gang's victims was David Morley, a bar manager who died following a savage beating on the south bank of the River Thames in London.
O'Mahoney was only 14 when she filmed part of the attack on 37-year-old Mr Morley on her state-of-the-art mobile phone, mirroring the so-called "happy slapping" craze. As he lay on the ground, she kicked his head like a football.
The other youths in the gang, Reece Sargeant, 21, Darren Case, 18, and David Blenman, 17, were each jailed for 12 years.
Following the sentencing at the Old Bailey, detectives released graphic CCTV footage of the gang's final attack on the night of Mr Morley's death.
The grainy footage, which had been played to the jury during their trial, showed the youths assaulting a man sleeping rough in a doorway in Leake Street, close to Waterloo station. It also showed O'Mahoney holding her mobile phone up in the air, apparently filming the attack.
The victim, Wayne Miller, was repeatedly kicked in the head before the group ran off "whooping" in celebration.
Detective Chief Inspector Nick Scola, who led the investigation, said: "When you first see the CCTV you are stunned by it. It takes a little time to get used to what you are seeing."
Sentencing the four, The Common Serjeant of London, Brian Barker, said the gang had filmed their assaults for "gratification", "amusement" and "enhanced status."
"You called this 'happy slapping' no victim on the receiving end would dignify it with such a deceptive description," he said.
"No-one listening to this case could fail to have been affected by your selfishness and blindness to the suffering of others. You sought enjoyment from humiliation and pleasure from the infliction of pain."
The four, all from Kennington, south London, stood impassively in the dock and showed little reaction as they were sentenced and told they would each have to serve at least two-thirds of their terms.