Eleven core samples of Ordovician carbonates and shale from within and nearby the Serpent Mound (Ohio) cryptoexplosion structure were analyzed for Re-Os isotopes. Whole-rock samples span a large range in measured 187Os/188Os (0.714 to 6.083), and exhibit a linear correlation of 187Os/188Os with 187Re/188Os. The linear relationship is inconsistent with a meteorite-crust mixing trend, but could represent an errorchron with an age of 485 ¿ 35 Ma, and an initial 187Os/188Os of 0.54 ¿ 0.15. In order to assess whether the errorchron slope and initial 187Os/188Os ratio may provide constraints on depositional age and the 187Os/188Os ratio of Ordovician seawater, acetic acid and HCl leaching studies were done on selected samples. The 87Sr/86Sr ratios of the leachates vary from 0.7081 to 0.7091, within the range found in Middle-Upper Ordovician brachiopods <Shields et al., 2003>, and consistent with the leachable fraction of the carbonates representing Ordovician seawater compositions. 187Os/188Os signatures for the leached samples are generally lower than those of respective whole rocks. However, five of six leachate samples produce a strong linear correlation on a Re-Os isochron diagram, the slope of which reflects an age of 443 ¿ 34 Ma, consistent with biostratigraphic constraints on the depositional ages of the sampled rock units (~445--472 Ma). The calculated initial 187Os/188Os of 0.53 ¿ 0.04 is distinctly lower than present-day seawater, and, if representative of Ordovician seawater, may reflect relatively low continental weathering rates due to waning Pan African orogenies as proposed to explain the slow decrease in seawater 87Sr/86Sr starting in early Ordovician <Qing et al., 1998>.