[2] O'Brien played first-class cricket for Cambridge University until 1956, making a total of 35 appearances.
[5] In addition to his 35 first-class matches for Cambridge University, O'Brien also played four for Ireland to 1958,[2] scoring 164 runs and registering one half century.
[3] He also made one first-class appearance for the Marylebone Cricket Club against Cambridge University in 1958.
[1] O'Brien undertook his National Service with the Royal Army Ordnance Corps while at Cambridge, serving with the rank of Second Lieutenant.
[6] He left Cambridge without achieving a degree, but nonetheless was employed in the city by Henry Gardner and Co Ltd.[1] O'Brien fell ill in 1959 and was diagnosed with leukemia.