A visual assay for the detection of heparinase was developed on the basis of a ternary system of Hg2+-heparin-osmium nanoparticles (OsNPs). First, heparin-capped OsNPs (heparin-OsNPs) were synthesized by a facile reduction method using heparin as the protecting/stabilizing agent. The oxidase-like activity of heparin-OsNPs, however, turned out to be low, which somewhat limits their application. We discovered that Hg2+ can significantly/specifically boost the oxidase-like activity of heparin-OsNPs via electrostatic interaction. The oxidase-like activity of heparin-OsNPs toward the oxidation of the substrate, 3,3',5,5'-tetramethylbenzidine, by dissolved O2 was found to increase by 76-fold in the presence of Hg2+. More significantly, heparin in heparin-OsNPs could be specifically hydrolyzed into small fragments in the presence of heparinase, which resulted in the weakening of the oxidase-like activity of Hg2+/heparin-OsNPs. On the basis of these findings, a linear response of the sensor for heparinase was obtained in the range 20-1000 μg/L with a low detection limit (15 μg/L), which is comparable to those of other reported sensors. Further, the colorimetric sensor was employed for the detection of heparinase in human serum samples with satisfactory results. We speculate that combining such surface modification of the osmium nanozyme with a sensing element could be an interesting direction for promoting nanozyme research in medical diagnosis.