Olivine-rich lavas in the Margi area, Cyprus, are part of the boninitic Upper Pillow Lava series of the Troodos ophiolite. Re-Os isotope compositions of both whole rock and olivine phenocrysts of these lavas have been measured to determine the initial ¹⁸⁷Os/¹⁸⁸Os isotope compositions of the primary boninitic magma. The Margi lavas have high and variable whole rock Re-Os isotope compositions (¹⁸⁷Re/¹⁸⁸Os: 1.060–8.613; ¹⁸⁷Os/¹⁸⁸Os: 0.1307–0.1599), with (⁸⁷Sr/⁸⁸Sr)_90Ma (0.705936 ± 0.001021), ε_Nd(90 Ma) (5.6 ± 2.5), and δ¹⁸O (8.59 ± 0.38 ‰). In contrast, olivine phenocrysts, which host spinel and melt inclusions, have low and constant ¹⁸⁷Os/¹⁸⁸Os from 0.1301 to 0.1336 and low ¹⁸⁷Re/¹⁸⁸Os from 0.1350 to 1.557. Initial ¹⁸⁷Os/¹⁸⁸Os of these olivine phenocrysts (0.1300 ± 0.0041) is lower than that of whole rock (0.1373 ± 0.0135). These facts indicate that the olivine phenocrysts faithfully record initial ¹⁸⁷Os/¹⁸⁸Os of the primary boninitic magmas, which are comparable to the ¹⁸⁷Os/¹⁸⁸Os of primitive mantle, whereas whole rock Re-Os isotopes have been affected by secondary processes after olivine crystallization. Further geochemical modeling indicates that, although Os can be mobilized during slab dehydration, the contribution of crustal Os is insufficient to significantly raise the ¹⁸⁷Os/¹⁸⁸Os of the mantle wedge in a nascent subduction zone. Overall, our findings provide new insights about Os recycling in the early stages of subduction.