The Southern United head coach has resigned from his role and will return to Ireland with his family this week.
He will be replaced by assistant coach Terry Boylan for the remainder of the season.
O’Reilly has spent just shy of four seasons with the club, having taken it to fifth-placed finishes in each of the last two.
He apologised for the timing of the move, saying he wished he could have seen out the season.
However, he had a job lined up, working in sports, and he could not extend the start date further.
There were many reasons for the move — both personal and professional — although they came back to wanting stability for his family.
Having been away from home for five years, they were away from family support.
Likewise with two young girls — the oldest starting school soon — he did not want to put them into school and then move again.
He added the cost of living in Dunedin was increasing and that was becoming challenging on a single income.
From a football perspective a lack of certainty was also evident.
O’Reilly had loved his time with Southern, but not knowing where the club or the league would be in the future made it hard to commit.
He felt the challenges facing Southern were "mammoth", particularly financially.
While he and the club had been trying to work through things for the past 15 months, it had reached a point where they could not reach an agreement.
He said he was leaving on good terms.
The club was just not in a position to take the next step in what he wanted to do, and to sustain that.
He said he was accepting of that, however, he also needed more certainty to justify staying here with his family.
From a league perspective it was a similar case.
Many questions still remained around New Zealand Football’s competitions review and how the league would look in the future.
He said the environment had been one which changed significantly every year and that contributed to the lack of stability.
"It’s probably been one of my biggest frustrations. I’ve never known an environment to change so quickly and so often," he said.
"The New Zealand national league, every year there seems to be a total upheaval.
"For me with a young family living a long way from home, it kind of becomes this is just not settling down.
"I think the club will be going through more change, the whole league will be.
"There’s a whole new review to work out, there’s a whole new structure to work out.
"It was probably just at that stage for me where it wasn’t settled enough and I had to make the decision for both those personal and professional reasons really."
Southern chief executive Chris Wright said O’Reilly had done an "incredible job" and the club would "miss him a huge amount".
