A new series of quinolinolate osmium carbonyl complexes were synthesized and characterized by spectroscopic methods. Single-crystal X-ray diffraction studies indicate that these complexes consist of an octahedral ligand arrangement with one chelating quinolinolate, one tfa or halide ligand, and three mutually orthogonal terminal CO ligands. Variation of the substituents on quinolinolate ligands imposes obvious electronic or structural effects, while changing the tfa ligand to an electron-donating iodide slightly increases the charge density on the central osmium atom. These Os(II) complexes show salient dual emissions consisting of fluorescence and phosphorescence, the spectral properties and relaxation dynamics of which have been studied comprehensively. The results, in combination with the theoretical approaches, lead us to propose that the emission mainly originates from the quinolinolate pi pi* state. Both experimental and theoretical approaches generalize various types of intersystem crossing versus those of the tris(quinolinolate) iridium Ir(Q)(3), and their relative efficiencies were accessed on the basis of the associated frontier orbital configurations. Our results suggest that <(1)d(pi)pi*vertical bar H(SO)vertical bar(3)pi pi*> (or <(3)d(pi)pi*vertical bar H(SO)vertical bar(1)pi pi*>) in combination with a smaller Delta ES1-Tt, gap (i.e., increasing the MLCT (d(pi)pi*) character) is the main driving force to induce the ultrafast S-1 -> T-1 intersystem crossing in the third-row transition metal complexes, giving the strong phosphorescent emission.