•Floral phenology and fruit set fail when olive tree growing under field conditions are exposed to 4 ºC above ambient temperature.•Olive floral differentiation processes seem to be affected by warmer temperatures favoring pistil abortion.•Pistil abortion along with fruit set reduction by heat may impact olive production.•The occurrence and duration of olive flowering processes will be advanced and extended under warmer temperatures. The increase in air temperature associated to climate change is expected to affect olive (Olea europaea L.) tree fructification. To determine the potential effects of global warming on floral phenology and fruit set under field conditions, olive trees growing in a Mediterranean climate type area were subjected to warmer temperature than ambient by the use of temperature controlled Open-top-Chambers (OTCs). Each OTC, equipped with heating and ventilation devices, was able to maintain a day/night temperature gradient between the tree and the surrounding environment of 4 °C throughout the complete reproductive cycle of this species. After three years of study, the results obtained have shown that increasing temperature 4 °C above the actual ambient temperature may led to an advance of the date of flowering in the olive, an extent of the flowering period, an increase of pistil abortion, and a reduction in fruit set, conditions which may reduce yield.