We report for the first time a complete X-ray diffraction in-situ crystal-structure refinement of a single crystal of olivine still trapped in a diamond (Udachnaya kimberlite, Siberia). A two-step experimental procedure, consisting of accurate crystal centering using a four-circle diffractometer equipped with a point detector and subsequent collection of complete intensity data using a second diffractometer equipped with a CCD detector, allowed us to overcome previously reported experimental problems and to refine the crystal structure without extracting the inclusion from the diamond host. The data allowed us to obtain the cation distribution over the two crystallographic M2 and M1 sites, which provided composition of olivine inclusion as Fo(92.7(4)). A novel experimental calibration of the pressure-volume equation of state for such composition was obtained using new in situ high-pressure X-ray data on a Fo(92) olivine single-crystal and new established compositional effects on olivine unit-cell volume. Such equation of state allowed us to determine the internal pressure at the olivine inclusion, P(i) = 0.40(1) GP a. The value for the internal pressure compares well with but has lower uncertainty than estimates obtained using micro-Raman spectrometry for similar olivine inclusions in diamonds from the same kimberlite. Taking into account elastic relaxation of the diamond-olivine pair to ambient P-T, we determined formation pressures of 3.5 GPa to 4.9 GPa, depending on the assumed temperature (800 degrees C to 1300 degrees C). These values suggest formation near the graphite-diamond boundary and are comparable to estimates from conventional and Raman thermobarometry for other peridotitic inclusions in Siberian diamonds.