O`Brien bettered former Matthew Hayden`s earlier World Cup record of 66-ball hundred which the former Australian opener had hit against South Africa in St Kitts in the 2007 edition in the West Indies.
27-year-old O`Brien reached the milestone by taking a double off Michael Yardy in the 41st over of Ireland run chase after England had batted first and posted 327 for eight.
The right-handed middle-order batsman hit 13 fours and six sixes in his explosive innings.
O`Brien`s record is the sixth fastest overall in ODIs.
Shahid Afridi holds the record of fastest hundred in ODIs in 37 balls. He also hit another ton in 45 balls. Others who have hit faster centuries in ODIs than O`Brien are Mark Boucher (44 balls), Brian Lara (45) and Sanath Jayasuriya (48).