The shift from denser forests to open, grass-dominated vegetation in west-central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published (n=2065) and new (n=173) oxygen isotope measurements (δ18O) in authigenic clays and soil carbonates to test the hypothesis that the OHT is linked to increasing wintertime aridity. Oxygen isotope ratios in meteoric water (δ18Op) vary seasonally, and clays and carbonates often form at different times of the year. Therefore, a change in precipitation seasonality can be recorded differently in each mineral. We find that oxygen isotope ratios of clay minerals increase across the OHT while carbonate oxygen isotope ratios show no change or decrease. This result cannot be explained solely by changes in global temperature or a shift to drier summers. Instead, it is consistent with a decrease in winter precipitation that increases annual mean δ18Op (and clay δ18O) but has a smaller or negligible effect on soil carbonates that primarily form in warmer months. We suggest that forest communities in west-central North America were adapted to a wet-winter precipitation regime for most of the Cenozoic, and they subsequently struggled to meet water demands when winters became drier, resulting in the observed open habitat expansion.