The olive tree (Olea europaea L.), a species adapted to the Mediterranean climate, has expanded into new climatic regions. Uruguay has a humid temperate climate and highly irregular climatic conditions among years. Environmental factors can be an obstacle to full production, as they affect pollination and fruit set. In this research, the phenology of five olive cultivars (Arbequina, Arbosana, Manzanilla, Picual and Koroneiki) widely cultivated in Uruguay was studied, using the BBCH phenological scale. The pollen grains of five cultivars were submitted to in vitro germination and incubated at temperatures of 0°, 10°, 15°, 20°, 25°, 30° and 40°C for 24 hours, and pollen grains germination and pollen tube length were evaluated. These temperatures were analyzed for the occurrence probability. They were based on the historical series analysis of hourly average temperature data, from September 26 to November 15 (flowering period), for the years of 1998 to 2019. Phenology results show that flowering can occur from day 268 to day 320 of the year, with cultivar 'Manzanilla' being the earliest and 'Arbosana' the latest. It was observed that the temperature of 10°C has no effect on pollen grain germination and on pollen tube growth. Moreover, the temperatures of 15 to 20°C are not very effective and the optimal temperature occurs between 25 and 30°C. In Uruguay, during flowering, temperatures between 10 and 20°C are more likely to occur. These results indicate possible causes that affect fruit set and productivity of olive trees in the field.