The purpose of this work was to develop an accurate osmium (Os) solution standard that is traceable to the International System of Units, the SI. A gravimetric reduction (GR) method was developed to accurately assay the Os in ammonium hexachloroosmate salt, (NH4)2OsCl6, the chosen starting material for the Os solution standard. This GR method was also applied to -200 mesh high-purity Os metal powder, which served as an independent source of Os for comparison. An alkali fusion method was developed to create water-soluble salts from the Os metal GR products of both the (NH4)2OsCl6 and -200 mesh high-purity Os metal powder. Quantitatively prepared Os solutions from each of these water-soluble salts were compared by inductively coupled plasma spectrometry, ICP-OES. The purities of the Os starting materials were determined by quantitatively performed trace metallic impurities analysis, by ICP-OES and ICP-MS; the O, N, and H impurities were determined by inert gas fusion analysis. The percent purities of the starting materials were applied as corrections to the Os assay results obtained by GR. The agreement between these Os solutions, corrected for random and systematic errors by error budget analysis, confirmed the accuracy of the Os assay in the (NH4)2OsCl6. The SI traceability of the Os assay in the (NH4)2OsCl6 salt was established through the GR procedure and the purity analysis of this material. An SI traceable Os solution standard was gravimetrically prepared from this batch of (NH4)2OsCl6, based on the accurate Os assay and the percent purity of the starting material.