The Os-isotope compositions of platinum-group minerals (PGMs) in ophiolite chromitites are commonly regarded as resistant to fluid-related processes, and have been used to track the evolution of Earth's convecting mantle. However, we have found significant differences in ¹⁸⁷187Os/¹⁸⁸Os between primary and secondary PGMs from metamorphosed ophiolite chromitites of the Dobromirtsi Ultramafic Massif, in the Central Rhodope Metamorphic Core Complex of southeastern Bulgaria. Primary (magmatic) PGMs hosted in unaltered chromite cores have ¹⁸⁷Os/¹⁸⁸Os from 0.1231 to 0.1270, and ¹⁸⁷Re/ ¹⁸⁸Os ≤ 0.002. T MA and T RD model ages, calculated relative to the Enstatite Chondrite Reservoir, cluster around three main peaks: ca. 0.3, 0.4, and 0.6 Ga. Secondary PGMs, produced by alteration of magmatic PGMs, have a wider range of variation (¹⁸⁷Os/¹⁸⁸Os = 0.1124-0.1398, ¹⁸⁷Re/¹⁸⁸Os ≤ 0.024); these grains yield T MA and T RD model ages from -1.7 Ga up to 2.2 Ga. The larger range in ¹⁸⁷Os/¹⁸⁸Os in the secondary PGMs is interpreted as due to reactions between the primary PGMs and infiltrating metamorphic-hydrothermal fluids with a range of Os-isotope compositions. This redistribution of Os in PGMs during metamorphism has significant implications for the interpretation of both whole-rock and in situ Os-isotope data in mantle-derived rocks.