The former Pirates midfielder headed powerfully home from David Buchanan’s volleyed cross after 49 minutes to settle an energetic League Two clash played in warm sunshine.
O’Toole had been roundly booed by home fans for choosing to leave their club at the end of the 2013-14 season but he came up with the perfect response.
Both sides squandered a gilt-edged first-half chance. Marc Richards broke clear for Northampton after eight minutes only to shoot wide with just goalkeeper Steve Mildenhall to beat.
That miss was matched by Rovers’ Matt Taylor after 36 minutes when he ran onto a poor back-header from the otherwise immaculate Richard Cresswell and rounded goalkeeper Adam Smith only to inexplicably delay his shot and lose possession.
A second half of few opportunities saw Mildenhall save from Cresswell and Richards to prevent further Northampton goals.