Osmium (₇₆Os) has seven naturally occurring isotopes, five of which are stable: ¹⁸⁷Os, ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and (most abundant) ¹⁹²Os. The other natural isotopes, ¹⁸⁴Os, and ¹⁸⁶Os, have extremely long half-life (1.12×10¹³ years and 2×10¹⁵ years, respectively) and for practical purposes can be considered to be stable as well. ¹⁸⁷Os is the daughter of ¹⁸⁷Re (half-life 4.12×10¹⁰ years) and is most often measured in an ¹⁸⁷Os/¹⁸⁸Os ratio. This ratio, as well as the ¹⁸⁷Re/¹⁸⁸Os ratio, have been used extensively in dating terrestrial as well as meteoric rocks. It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons. However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the Cretaceous–Paleogene boundary that marks the extinction of the dinosaurs 66 million years ago.

There are also 31 artificial radioisotopes,^[5] the longest-lived of which is ¹⁹⁴Os with a half-life of six years; all others have half-lives under 93 days. There are also ten known nuclear isomers, the longest-lived of which is ^191mOs with a half-life of 13.10 hours. All isotopes and nuclear isomers of osmium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

The isotopic ratio of osmium-187 and osmium-188 (¹⁸⁷Os/¹⁸⁸Os) can be used as a window into geochemical changes throughout the ocean's history.^[6] The average marine ¹⁸⁷Os/¹⁸⁸Os ratio in oceans is 1.06.^[6] This value represents a balance of the continental derived riverine inputs of Os with a ¹⁸⁷Os/¹⁸⁸Os ratio of ~1.3, and the mantle/extraterrestrial inputs with a ¹⁸⁷Os/¹⁸⁸Os ratio of ~0.13.^[6] Being a descendant of ¹⁸⁷Re, ¹⁸⁷Os can be radiogenically formed by beta decay.^[7] This decay has actually pushed the ¹⁸⁷Os/¹⁸⁸Os ratio of the Bulk silicate earth (Earth minus the core) by 33%.^[8] This is what drives the difference in the ¹⁸⁷Os/¹⁸⁸Os ratio we see between continental materials and mantle material. Crustal rocks have a much higher level of Re, which slowly degrades into ¹⁸⁷Os driving up the ratio.^[7] Within the mantle however, the uneven response of Re and Os results in these mantle, and melted materials being depleted in Re, and do not allow for them to accumulate ¹⁸⁷Os like the continental material.^[7] The input of both materials in the marine environment results in the observed ¹⁸⁷Os/¹⁸⁸Os of the oceans and has fluctuated greatly over the history of our planet. These changes in the isotopic values of marine Os can be observed in the marine sediment that is deposited, and eventually lithified in that time period.^[9] This allows for researchers to make estimates on weathering fluxes, identifying flood basalt volcanism, and impact events that may have caused some of our largest mass extinctions. The marine sediment Os isotope record has been used to identify and corroborate the impact of the K-T boundary for example.^[10] The impact of this ~10 km asteroid massively altered the ¹⁸⁷Os/¹⁸⁸Os signature of marine sediments at that time. With the average extraterrestrial ¹⁸⁷Os/¹⁸⁸Os of ~0.13 and the huge amount of Os this impact contributed (equivalent to 600,000 years of present-day riverine inputs) lowered the global marine ¹⁸⁷Os/¹⁸⁸Os value of ~0.45 to ~0.2.^[6]

Os isotope ratios may also be used as a signal of anthropogenic impact.^[11] The same ¹⁸⁷Os/¹⁸⁸Os ratios that are common in geological settings may be used to gauge the addition of anthropogenic Os through things like catalytic converters.^[11] While catalytic converters have been shown to drastically reduce the emission of NO_x and CO₂, they are introducing platinum group elements (PGE) such as Os, to the environment.^[11] Other sources of anthropogenic Os include combustion of fossil fuels, smelting chromium ore, and smelting of some sulfide ores. In one study, the effect of automobile exhaust on the marine Os system was evaluated. Automobile exhaust ¹⁸⁷Os/¹⁸⁸Os has been recorded to be ~0.2 (similar to extraterrestrial and mantle derived inputs) which is heavily depleted (3, 7). The effect of anthropogenic Os can be seen best by comparing aquatic Os ratios and local sediments or deeper waters. Impacted surface waters tend to have depleted values compared to deep ocean and sediments beyond the limit of what is expected from cosmic inputs.^[11] This increase in effect is thought to be due to the introduction of anthropogenic airborne Os into precipitation.

The long half-life of ¹⁸⁴Os with respect to alpha decay to ¹⁸⁰W has been proposed as a radiometric dating method for osmium-rich rocks or for differentiation of a planetary core.^[2][12][13]