Olusanya threatened early when he made a great run and was practically one-on-one with Jack Butland but watched his effort roll past the left-hand post.
Against the run of play, it was Rangers who led.
Nicolas Raskin's initial shot was saved by Ellery Balcombe but the ball was popped back out straight into the feet of Diomande, who smashed it beyond the keeper.
Gogic levelled things up when he powerfully turned in Mark O'Hara's perfectly delivered corner after rising highest in a crowded area.
It was an improved Rangers performance in the second period, but it was the visitors who thought they had taken the lead when Olusanya slotted beyond Butland.
Olusanya ran clear of Robin Propper to net but, fortunately for the defender, the flag was eventually raised for offside and Propper later made way for Rangers captain James Tavernier.
Dessers had also come off the bench and the forward burst past St Mirren's defence to set up Cerny's fifth goal in three games, a low and firm shot.