In an early-November contest at London Stadium, Saïd Benrahma's early opener was cancelled out by Wilfried Zaha's clinical finish in the first-half – and Palace looked set to depart east London with a well-earned point.
But Olise – who had offered plenty of threat from the right-hand side all afternoon – had other ideas.
With West Ham conceding possession in the final seconds of stoppage time, Palace set into motion a sharp counter-attack which saw Zaha spray the ball out to Olise.
The No. 7 squared up Aaron Cresswell, took a few touches to work it onto his left foot and launched a deflected effort high over Łukasz Fabiański and in at back post before wheeling away in ecstasy.