The 5d transition-metal impurity, osmium, has been incorporated during the growth of n-type GaAs epitaxial layers using low-pressure metal-organic chemical-vapor deposition to characterize defect states associated with this heavy and, therefore, thermally stable dopant impurity. Deep-level transient spectroscopy has been employed to investigate the electrical characteristics of Os-related deep levels. Two prominent deep levels have been identified with Os at Ec−0.28 eV (Os1) and Ec−0.41 eV (Os2) in the upper half-band-gap of GaAs, while no Os-related level has been clearly detected in the lower half-band-gap. The detailed characteristics determined for the two levels include thermal emission rate signatures, leading to the above cited thermal activation energies, electron-capture cross sections, and their temperature dependence, measured by direct pulse-filling technique and deep-level concentrations. Further, both levels are found to exhibit a significant dependence of thermal emission rates on the junction electric field. While Os1 does not allow accurate quantitative investigations of this field dependence due to its relatively low concentration, detailed quantitative data on the field dependence of the level Os2 are reported. Analysis of these data in light of the available theoretical models allows us to conclude that Os2 is probably a substitutional donor-type defect in GaAs.