Understanding the fast Li ionic conductors of oxygen-substituted thiophosphates is useful for developing all-solid-state batteries because these compounds possess a high electrochemical stability and thus may be applied as solid electrolytes. In this study, we synthesized the Li9+δP3+δ'S12-kOk series of solid solution phases with the same structure as the Li10GeP2S12 superionic conductor and characterized their crystallinity, solid solution range, and chemical stabilities. Two methods (mechanochemical and melt quenching) were used for sample synthesis. Mechanochemical synthesis was used to obtain samples within a wide range of sulfur/oxygen substitution degrees, and the solid solution range was determined to be 0 < k ≤ 3.6 based on their lattice parameter variation. Meanwhile, the melt-quenched Li9P3S9O3 phase exhibited a high degree of crystallinity up to its particle surface and was thus selected for neutron crystal structure analysis, which revealed the oxygen distribution related to the solubility limit. The highly crystalline melt-quenched Li9P3S9O3 showed better stability in the air atmosphere compared to the mechanochemically synthesized counterpart with a low crystallinity, implying that sample crystallinity is an important parameter in evaluating the air stability of thiophosphates. The promising electrochemical properties of the solid solution series were demonstrated by the stable charge-discharge cycling of an all-solid-state lithium metal cell using the Li9+δP3+δ'S12-kOk electrolyte with k = 0.9 and a conductivity of >1 × 10-3 S cm-1 at 300 K.