Surprise package Scunthorpe, whose flying start to the season saw them arrive at Oakwell in fourth spot, almost caught the home side out after just four minutes.
Barnsley were dithering at the back when Ian Baraclough latched onto the ball and fired in a dipping long-range effort that drifted high over the bar.
But in the only other first-half chance the Iron managed to produce Tykes defender Anderson Silva's sloppy header sent in leading scorer Martin Paterson but Heinz Muller saved with his legs.
In between times Barnsley hardly troubled Scunthorpe keeper Joe Murphy. Hungarian frontman Istvan Ferenczi was off target when he snapped in a shot from a loose ball.
A super ball from Silva was aimed at Odejayi but a superb block from Marcus Williams kept the Barnsley striker out.
But in the dying seconds of the first half Barnsley went ahead with their freakish goal. A hopeful ball into the area was chased down by Jamal Campbell-Ryce in his first start since his £175,000 arrival from Southend on transfer deadline day.
Defender Andy Crosby appeared to be favourite to clear the danger but he smashed the ball against team-mate Williams and the ball looped high into the air and over keeper Murphy who had drifted off his line.
Barnsley could have stretched their lead nine minutes into the second half when a free-kick from Silva was whipped into the area and Ferenczi found space to produce a glancing header that beat Murphy only for the ball to smack against the bar.
And when a loose ball was picked up by Dominik Werling he unleashed a fearsome 30-yard drive that Murphy punched out but only as far as Werling for a carbon-copy effort that the keeper held onto.
But after 65 minutes it was the home side that made sure of the points with an impressive goal from Odejayi.
A ball from Stephen Foster put Scunthorpe skipper Crosby under pressure and his attempted clearance cannoned off Odejayi who galloped 40 yards, shrugged off Andy Butler and rifled in a low shot.