O'Reily was highly regarded by many in South Australian society, with Adelaide's daily newspapers praising his character, administrative ability and positive relations with non-Catholics.

O'Reily was born John O'Reilly on 19 November 1846, in Kilkenny, Ireland, the son of Michael, a military officer, and Anne, née Gallagher.

The poor financial state of the Archdiocese made the discussions contentious, with O'Reilly concerned about the amount of debt Port Augusta would inherit from Adelaide.

To ensure a sufficiently large population for the new diocese, Moran suggested that its boundaries be altered from those drawn up by the Pope, but Reynolds opposed this plan.

[5] Dissatisfied with the outcome of the discussions, O'Reilly decided to travel to Sydney to deliberate further with the Cardinal, prompting Reynolds to write to Moran: "If the Holy See would think well of giving the entire colony of South Australia to the care of Dr O'Reily,[sic] I would offer no opposition to it and would gladly enter a monastery and spend my few remaining years in preparation for the end!

"[5]On 27 October, O'Reilly wrote to Pope Leo XIII requesting permission to reverse his decision to accept the appointment to Port Augusta, believing he would be unable to administer a diocese with such significant debt and such a small, impoverished population.

[5] The concerns O'Reilly raised prompted Cardinal Simeoni of the Congregation for the Propagation of the Faith to call for an official church inquiry into the financial situation of the South Australian dioceses.

At the bishop's expense, the order's Mother-General spent two months in Port Pirie, and once she had found appropriate living quarters, a school was established, teaching over 100 students.

He authored a pastoral letter on the matter in March 1889, and the next year gave a speech criticising the lack of government funding for Catholic schools.

After suffering a two-year illness, Archbishop Reynolds died in June 1893,[15] and by January 1894, rumours of O'Reily being appointed as his successor were published in Port Augusta papers.

Having reduced the new diocese's debts by half in his six years as bishop, upon becoming Archbishop of Adelaide, O'Reily found the rest of the old diocesan liabilities awaiting him, their size having actually increased.

These sales, combined the efforts of the Archbishop and diocesan priests to secure donations from around the Archdiocese, meant that by 1901 O'Reily was able to report that the debt had been reduced by three-quarters.

At the referendum, South Australians affirmed the system of free secular education, but rejected scriptural readings in schools and the capitation grant.

[2] Increasingly, his episcopal duties were fulfilled by Bishop of Port Augusta John Norton, who would have to visit the more remote parts of O'Reily's see on his behalf.

[21] In an editorial in 1913, The Advertiser praised O'Reily, lauding his administrative abilities and crediting the good relations between Protestants and Catholics in South Australia to his "broadness of mind and to his quiet determination to avoid all unnecessary controversy.

"[22] Upon his death, The Register gave a glowing editorial, describing him as "loveable and highly estimable", a "hard hitter" and asserting that, had he not been a religious leader, he could have been "a leading financier, Imperial legislator or even an eminent military commander.