Preparation and characterization of a new class of emissive Os(II) complexes (15) have been achieved using 3-(thiazol-2-yl), 3-(benzothiazol-2-yl), 3-(imidazol-2-yl) and 3-(benzimidazol-2-yl) azole chelates, together with the incorporation of one diphosphine ancillary for satisfying the octahedral coordination arrangement. The resulting Os(II) complexes, except for 5, all show bright emission spanning visible region from green to saturated red, and their structural versus spectroscopic properties have been comprehended by absorption/emission together with computational approaches. Subsequently, a series of solution-processed OLEDs using 14 as the dopant were successfully fabricated, demonstrating excellent device performances with external quantum efficiency (EQE) of 15%, current efficiency exceeding 48 cd/A, and power conversion efficiency up to 50 lm/W. Complex 4 is the only Os(II) based phosphor so far suitable for the fabrication of highly efficient green-emitting OLEDs.