Olive reproductive phenology depends on the balance between winter chilling and subsequent heat accumulation, yet multi-environment Mediterranean observations to assess this dynamic across thermal regimes remain scarce. We monitored Full Budburst (FB) and Full Flowering (FF) during 2021–2024 in 670 cultivars across five germplasm banks in Greece, Italy, Morocco, Spain, and Türkiye. Median dates, periods, dispersion, and phenological stage overlap were related to bioclimatic factors and chilling and heat accumulation. The influence of the germplasm-bank environment, year, and their interaction on the timing of both stages was assessed. The Median Full Budburst (FB₅₀) and Median Full Flowering date (FF₅₀) were strongly correlated. Likewise, budburst dispersion (ID_bb) was positively associated with flowering dispersion (ID_ff), serving as descriptive indicators of phenological synchrony under varying thermal regimes. Warmer conditions between FB₅₀ and FF₅₀ were associated with a shorter transition to flowering. Conversely, warmer pre-budburst conditions and lower chilling accumulation were associated with greater budburst and flowering dispersion and reduced synchrony. These findings indicate that budburst provides an important temporal reference for flowering. Integrating budburst and flowering dispersion with transition gap (T_gap) metrics offers a quantitative framework for characterizing population-level phenological responses under contrasting thermal regimes and enhancing future olive breeding programs.