Michael was born on 2 June 1886 to Francis O'Brien and Mary Gertrude O'Brien (nee McSweeney) in Albury, NSW. He was one of 7 children, 4 brothers and 2 sisters. By 1904 the family was living in Geelong, where his mother remained until her death in 1941.

Father Michael was serving as a Catholic priest at St Patrick's Cathedral when he first enlisted in the Chaplain's Corps on 7 August 1916. He disembarked for the UK from Melbourne on the HMAT Ballarat A70. On arrival in the UK he reported for duty at thr 3rd Australian Hospital, Gartford, where he remained until his return to Australia in Febraury 1917 on th HT Benalla. During the voyage he was an duty as a Chaplain. He arrived back Melbourne in March 1917.

On 21 June, 1917 he embarked again from Melbourne on the HMAT Suevi A29, arriving in Liverpool 2 months later. By 18 September he embarked to France from Southhampton to take up his post with the 1st Division Troops at their hospital in Le Havre. In December he moved to the H.Q. 1st Division Train followed by a posting in April 1918 to the 1st Division Artillery Column. In June 1918 Michael moved to 1st Australian Casualty Clearing Station where he remained until he took leave to Nice on 7 September 1918. Two weeks later he returned to the 1st ACCS. In January 1919 he became attached to the 15th Battalion, returning to the London HQ AIF until he returned to Australia on 21 June, 1919 on the Konigin Luise.

Father Michael's mother, Mary, was there to meet him on his arrival back in Melbourne when he returned to work at at St Patrick's Cathedral until he was appointed to head the newly established  Armadale/Toorak parish of Our Lady of Lourdes, High Street, Prahran. The foundation stone for the Church and Presbytery was laid on 4 June 1922 by the Archbishop of Melbourne, Dr. Mannix. A temporary church and school were quickly constructed with the opening Mass celebrated on 8 October 1922. Father O'Brien was instrumental in fund raising for the new church with plans well developed by 1929 and foundations laid in early 1930. He remained the parish priest until his death in 1942.