We investigated a LaFeAsO single crystal by means of synchrotron Mössbauer spectroscopy under pressure up to 7.5 GPa and down to 13 K and provide a microscopic phase diagram. We found a continuous suppression of the magnetic hyperfine field with increasing pressure, and it completely vanishes at ∼7.5GPa, which is in contrast to the behavior in polycrystalline samples where the magnetic order vanishes at ∼20GPa. The different behaviors of the polycrystalline samples might be due to As vacancies. Our results are in qualitative agreement with density functional theory calculations where a reduction of the magnetic moment with increasing pressure was found. We found that among different samples the magnetic phase-transition temperature as well as the low-temperature magnetic hyperfine field decrease with increasing unit-cell volume.