Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge dynamics and the state of Earth’s upper mantle. Yet, interpreting anisotropy in the context of grain-scale deformation processes observed in laboratory and natural olivine samples has proven challenging due to the vast length scale differences. We bridge this observational gap by estimating the first in situ elastic tensor of oceanic lithosphere using compressional- and shear-wavespeed anisotropy observations, with fast azimuth parallel to the fossil-spreading direction. This observation is compared with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Findings indicate D-type olivine lattice-preferred orientation (LPO) with girdled [010] and [001] crystallographic axes and strain accumulation of 300–400%, challenging the prevailing assumption of A-type LPO. This LPO is consistent with olivine deformation during seafloor spreading via grain-size sensitive dislocation-accommodated grain boundary sliding (disGBS), rather than grain-size insensitive dislocation creep. Such deformation implies in situ grain sizes of 0.3–15 mm, smaller on average than steady-state predictions for pure olivine from laboratory calibrations, which may be attributed to grain-boundary pinning by secondary phases or an underestimate of the importance of disGBS by standard flow laws. This work demonstrates the ability to integrate laboratory- with seismological-scale observations of seismic anisotropy and provides new constraints on in situ grain size and associated rheology during near-ridge mantle deformation.