[3] Pope Pius IX appointed O'Reilly as the first bishop of the newly established Diocese of Springfield on June 18, 1870.

Mercy Hospital in Springfield developed from a Holyoke mission of the Sisters of Providence of St. Vincent de Paul.

[5] According to author Stephen Kiltonic, O'Reilly was described as tall with "very kind gray eyes that would twinkle with humor".

"The old folks, who remember him, speak of his quiet but convincing eloquence, his constant affability and kindness, his abiding trust in God that, to every cloud, gave a silver lining.

"[6] Author William Byrne described O'Reilly as a supportive manager of his priests, allowing them to make decisions on their own.