Jack O'Brien was an Australian rules football

Australian rules football

Australian rules football, officially known as Australian football, also called football, Aussie rules or footy is a sport played between two teams of 22 players on either...

er who played for Essendon

Essendon Football Club

The Essendon Football Club, nicknamed The Bombers, is an Australian rules football club which plays in the Australian Football League...

 and Fitzroy

Fitzroy Football Club

The Fitzroy Football Club, formerly nicknamed The Lions, is an Australian rules football club formed in 1883 to represent the inner Melbourne suburb of Fitzroy, Victoria and was a foundation member club of the Victorian Football League on its inception in 1897...

 in the Victorian Football League

Australian Football League

The Australian Football League is both the governing body and the major professional competition in the sport of Australian rules football...

 (VFL).

A premiership player in his debut season, O'Brien played from the back pocket

Back pocket

In Australian rules football, the back pocket refers to a position on the field deep in defence.Back pocket players, situated in the backline, need to have good spoiling skills and usually, quality back-pockets are noted for their hardness....

 in Essendon's 1912 VFL Grand Final triumph. He was also used as a follower and returned to his original club South Ballarat for the second half of the 1913 season. After playing at Fitzroy in 1917 and 1918, O'Brien crossed to Northcote

Northcote Football Club

Northcote Football Club, nicknamed The Dragons, was an Australian rules football club which played in the VFA from 1908 until 1987. The club's colours were green and yellow and it was based in the Melbourne suburb of Northcote...

in 1919.