On the overall balance of play the Cobblers were good value for their narrow win heading into Sunday's second leg at Whaddon Road, but the Robins will feel more than capable of turning the tie around and returning to Wembley 12 months after their final defeat to Crewe.
O'Donovan's well-taken finish, his sixth goal of the season, came midway through the first half from Chris Hackett's right-wing cross.
Burly striker Adebayo Akinfenwa was denied a precious second goal for Northampton when he saw his close-range header kept out by a tremendous one-handed save by Scott Brown.
Paul Benson spurned two gilt-edged chances for Cheltenham while Jermaine McGlashan could feel hard done by not to have been awarded a second-half penalty when he was brought down by Luke Guttridge, but the visitors failed to really trouble Lee Nicholls at Sixfields.
It was Northampton's first win in seven attempts over the Robins, who completed a league double this season, but the semi-final is far from over as both teams chase a return to England's third tier for the first time since 2009.
- Sports & Recreation
- Adebayo Akinfenwa