An Old Xaverian, O'Brien played for St Kilda's thirds and Under-19s sides before joining Collingwood.
[1] In the final round of the 1985 VFL season, O'Brien played his first and only game for Collingwood, against Carlton at Waverley Park.
[2] The following year he represented Victoria at the Amateur football carnival in Adelaide and kicked eight goals in the final to win the Hugh Millard Medal.
[2] This included a five-goal haul in a four-point win over St Kilda, which he won for his team with a soccered goal from 30 metres out late in the final quarter.
[4] He played just five times in 1990 but had a memorable game against the Sydney Swans at Carrara when he kicked a career best seven goals.