Daryl Murphy had headed the Tractor Boys into a half-time advantage, but O'Brien's introduction just prior to the break inspired the Reds to snatch a share of the spoils from an entertaining clash in Suffolk.
Aaron Cresswell's set pieces proved to be the biggest source of goalscoring chances in a first half that was dominated by Ipswich following a quiet opening quarter-of-an-hour.
The left-back's delivery found Luke Chambers in the centre of the goal, but the former Nottingham Forest centre-back headed over the crossbar from eight yards.
Murphy was more accurate with his nodded effort, though, as he shook off his marker and met Cresswell's 41st-minute corner at the far post to put the hosts ahead.
From then on, the Reds applied constant pressure and appeared the more likely team to grab a winner, with O'Brien and Jason Scotland going close in two of their 11 second-half efforts on Dean Gerken's goal.
Former Ipswich striker Scotland ruffled the side-netting after he had been allowed too much space to chest down and then unleash a half-volley from O'Brien's free kick.
Cresswell was sent off for his protests after the final whistle had blown on the 1-1 draw that extends Barnsley's unbeaten run to three games, with Ipswich now winless in five Championship matches.