A series of 2-aryloxazolo[4,5-f] [1,10]- phenanthroline ligands ((NN)-N-Lambda ligands) and their cationic iridium (III) complexes,(1-11, aryl = 4-NO2,phenyl (1), 4-Br-phenyl (2), Ph (3), 4-NPh2-phenyl (4), 4-NH2-phenyl (5), pyridin-4-yl (6), naphthalen-1=yl naphthalen-2-yl (8), Ohenanthren-9-y1 (9), anthracen-9-y1 (10), and pyren-1-yl (11)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (1-5), or different aromatic substituents with varied degrees of pi-conjugation (6-11) on oxazolo[4,54][1,10]phenanthroline ligand, we aim to understand the effects of terminal substituents at the (NN)-N-Lambda ligands on the photophysics of cationic Ir(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo[4,5-f][1,10]phenanthroline motif; while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f],[1,10]phenanthroline motif All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized irc (1)pi,pi*/(1) ILCT (intraligand charge transfer) transitions, mixed with (LLCT)-L-1 (ligand-to-ligand charge transfer)(MLCT)-M-/ (metal-to-ligand charge transfer) transitions. At the range of 350-570 nm, all complexes exhibited moderately strong 1ILCT/(LLCT)-L-1/(MLCT)-M-1 transitions at 350-450 nm, and broad but very weak (LLCT)-L-3/(MLCT)-M-3 absorption at 450-570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes 1 and 4 that contain NO2 or NM, substituent at the phenyl ring, respectively, all other complexes exhibited Moderate to strong triplet excited-state absorption in the spectral region of 440-750 nm. Moderate to very strong reverse -saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 approximate to 4 > 5 approximate to 10 approximate to 6 approximate to 8 > 1. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes.