Sligo Rovers' poor form continued at the Showgrounds as they were overturned by a well organised Finn Harps.
Kosovar Sadiki was the only player on the scoresheet as the Donegal side, who had Mark Coyle sent off late on, picked up their first league victory since May.
With just one victory in their last five in all competitions, Rovers were looking to make it three wins from three over their neighbours this season.
Teenage striker Johnny Kenny was offered an early opportunity to set the home side on their way. Harps defender Kosovar Sadiki let the ball slide under his feet, but the incoming Kenny could only fire straight into the legs of Mark McGinley from an angle after eight minutes.
Ollie Horgan’s charges have one of the poorest defensive records in the division to date and they offered up further chances to Sligo’s Kenny and Niall Morahan.
Morahan was unable to finish Jordan Gibson’s cross 13 minutes in. While the ball somehow stayed out nine minutes later.
Walter Figueira’s initial drive was blocked away by McGinley, before the ball ended up three yards from goal. With McGinley stranded, Kenny looked favourite to profit, but Ethan Boyle managed to clear the danger before Kenny could pounce.
The visitors went ahead with their first shot on goal two minutes into the second half.
O’Sullivan’s set-piece was headed in by Sadiki beyond McGinty, although an irate reaction from the hosts suggested there was a foul on a Rovers defender in the process.
Harps' Tunde Owolabi saw his header loop over the crossbar on the hour as they began to dictate the pace of the tie.
The away side then had Coyle dismissed minutes before the end of the 90 minutes, while McGinley pulled off a super save in the dying moments to deny Mark Byrne from point-blank range.
Sligo Rovers: Ed McGinty; Lewis Banks, Garry Buckley, John Mahon (Shane Blaney 86), Colm Horgan; Niall Morahan (Adam McDonnell 86), Greg Bolger; Walter Figueira, Jordan Gibson (Ryan De Vries 70), Romeo Parkes; Johnny Kenny (Mark Byrne 61).
Finn Harps: Mark McGinley; Ethan Boyle, Kosovar Sadiki, David Webster, Jordan Mustoe; Mark Coyle, Will Seymore; Karl O’Sullivan (Ryan Rainey 59), Barry McNamee, Dan Hawkins (Ryan Connolly 87); Tunde Owolabi (Sean Boyd 83).
Referee: Adriano Reale.