Significant imbalances in terrestrial water storage (TWS) and severe drought have been observed around the world as a consequence of climate changes. Improving our ability to monitor TWS and drought is critical for water-resource management and water-deficit estimation. We use continuous seismic ambient noise to monitor temporal evolution of near-surface seismic velocity, dv/v, in central Oklahoma from 2013 to 2022. The derived dv/v is found to be negatively correlated with gravitational measurements and groundwater depths, showing the impact of groundwater storage on seismic velocities. Seasonal cycling of dv/v follows atmospheric temperature changes with a phase shift, which can be explained by thermo-elastic strain in the uppermost crust and sedimentary cover. The occurrences of droughts appear simultaneously with the local peaks of dv/v, demonstrating the sensitivity of near-surface seismic velocities to droughts. The results illustrate the potential of using seismic data for monitoring TWS and drought at regional to local scales.