The Cobblers moved into the play-off positions for the first time this season while Darlo moved closer to the Blue Square Premier.
Northampton's biggest home league gate of the season saw Ian Sampson's side continue their hot streak which has seen them beaten just twice in their last 17 matches.
They were sent on their way in the 20th minute when defender Johnson converted the rebound after Adebayo Akinfenwa had struck the underside of the crossbar.
Osman made it 2-0 six minutes before the break when the visiting defence failed to cope with a corner and the midfielder followed up.
Cobblers keeper Jason Steele saved well from Andrew Milne and Josh Gray after the break to preserve his side's lead.
Source: Team Talk
Source: Team Talk