We report the electronic structure and thermoelectric (TE) properties of OsX2 (X: S, Se, Te), and find a giant value of thermopower of magnitude 600 μV K-1-800 μV K-1 for a wide temperature range of 100 K-500 K for hole doping (at 1018 cm-3), which is higher than the value found for well established TE materials. The optimized structural parameters are in good agreement with available experimental reports. The mechanical stability of all the compounds are confirmed from the computed elastic constants. The band gap of the investigated compounds is examined by several exchange correlation functionals, and TB-mBJ with modified parameters is found to be the best. The heavy valence bands stimulate the thermopower value for hole doping and light conduction bands intensifies the electrical conductivity values for electron doping, enabling both 'n' and 'p' type doping favourable for TE applications at higher concentrations (1020 cm-3), which brings out the device application. Our results unveil the possibility of TE applications for all the examined compounds for a wide temperature range (100 K-500 K), and OsS2 specifically is quite alternative with the performing temperature ranging from 100 K-900 K. © 2018 IOP Publishing Ltd.