This study seeks to find the primary influence on the seasonal cycle of the oxygen isotopic composition of precipitation (δ18Op) along the western U.S. coast. Observed long-term mean seasonal variations ofδ18Op from 16 different stations along the west coast are presented. The most robust features in the observations are high values in the summer and a drop in δ18Opduring the winter. The Isotope-incorporated Global Spectral Model (IsoGSM) also simulates this wintertime drop inδ18Op along the west coast of the U.S. Sensitivity experiments are performed with IsoGSM where individual oxygen isotope fractionation processes are turned off. These simulations reveal that the primary control on the seasonal variations is equilibrium oxygen isotopic fractionation during vapor condensation. There is almost no influence of the temperature dependence of equilibrium fractionation on the seasonal δ18Op cycle for both evaporation and condensation. Additional experiments (including tagging simulations) are performed to better understand why Rayleigh distillation causes the seasonal variation in δ18Op. The tagging simulations and budget calculations reveal that vertical oxygen isotope gradients and variations in condensation height cause the seasonal cycle in δ18Op. This results from seasonal changes in the polar jet, and subsequent changes to divergence and vertical velocities, which affects the uplift of moisture. These findings suggest that δ18Op in the western U.S. is a tracer of condensation height on seasonal timescales. The large influence of condensation height on δ18Op seasonality complicates interpretations of interannual climate proxy records based on isotopes in precipitation as the seasonality is likely not static.