O'Malley, Queenstown Lakes District Council's infrastructure services strategic project manager, left for Nepal in October to acclimatise ahead of the event, the world's highest marathon.
In October, the Otago Daily Times reported O'Malley was attempting the 42km race to raise money and awareness for Jigsaw Central Lakes.
Jigsaw is a national network of more than 40 independent, community-based, social service organisations working to stop child abuse, neglect and family violence.
O'Malley said at the time it was "almost a taboo topic that people don't want to hear about".
"What I would love to do for Jigsaw Central Lakes is get the awareness out there. Hopefully, that will be really good for them and the people they help."
In an email to the ODT, received late on Tuesday night, O'Malley reported he had finished the gruelling race in 33rd position, in a time of 6hr 51min.
"There was a small crowd at the finish line and I was a happy camper to be at the end of this gruelling run," O'Malley (37) said in his email.
"You can see the relief and delight in my face for the finish line photo. I'm not sure I'd ever take this beast on again, but also realise time eases the leg muscles.
"I finished in 33rd place overall, one better than I was told, in a time of 6hr 51min ... there was a course record set that day, 4hr 32min. Unbelievable."
However, the race was not all plain sailing. O'Malley was affected by altitude just 100m into the race, describing it as "breathing like a large man trying to get out of a small hole with a plastic bag on his head".
"My sides ached, lungs burning, I was sure the onset of a massive heart attack was exactly like this.
"So, 200m into the run, I pulled to the side pretending I needed a [bathroom break], gasping lungfuls of cold, low oxygen air.
"The joys of running at 5100m altitude."
About the 25km mark, O'Malley was listening to Meatloaf's Bat Out of Hell on his iPod when he was "overtaken by a monk".
"I couldn't believe it.
"This dude, with prayer beads in one hand and flip flops passed me ... talk about soul destroying.
"Then Lou Reed came on my iPod with the song Perfect Day and I thought 'actually, yes ... this is a perfect day, despite being overtaken by a praying monk'."
Arrowtown's Ken Hardman (60) also finished the race, in 9hr 45min.
