Cross-correlation function of each observation with an OH model (black-and-white colour map) along with the expected WASP-121 b velocities (black dashed line) compared to where tellurics are in velocity space (blue dotted line). The cross-correlation maps are shown in the BERV-corrected frame. In none of our observation nights do the planet velocities overlap with where the telluric are, lowering the risk that any remaining telluric residuals affect our results.