The Cobblers were quickly out of the blocks and Clive Platt was foiled by Brian Saah's last-gasp challenge while Chris Hackett had a close-range effort blocked.
O'Donovan saw his low drive saved by Michael Poke and Ishmel Demontagnac fired over at the end of Northampton's best move.
In between Jordan Chappell tested Lee Nicholls and Ryan Jarvis fired wide but Poke did well to foil Ben Harding following a scramble as Northampton dominated the first half without getting their reward.
The Cobblers picked up from where they left off after the restart. Ben Harding hit a post, Hackett was denied by Poke from the rebound and from the ensuing corner John Johnson headed just wide.
O'Donovan had a header ruled out for offside but Torquay had a strong penalty appeal waved away when Elliot Benyon was tripped by Clarke Carlisle.
But with five minutes remaining Hackett turned inside before crossing for O'Donovan to head past Poke from 10 yards.
Rene Howe went close to an equaliser moments later and Nicholls denied Benyon in the last minute.