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Probing the electron hybridization between alkaline metal and oxygen ions is fundamentally important to understand variable emergent phenomena in alkaline titanates, however, experimental study of which is very limited. In this letter, in order to characterize the evolution of element-specific electronic structures in single crystal alkaline titanates ATiO3 (A = Ca, Sr, Ba), we carried out resonant soft X-ray absorption spectroscopy at Ti L2,3- and O K-edges and performed multiplet calculations. In sharp contrast to analogous Ti electronic structures in ATiO3, the electronic structure of oxygen changes drastically with different A-site metals (Ca, Sr, Ba), indicating the strong hybridization between alkaline metal and oxygen ions. Our work reveals an important role of A-site alkaline metals in determining the electronic structures of oxygens in ATiO3, and paves a way to design novel quantum states (e.g., coexisting ferroelectricity, ferromagnetism, and superconductivity) with oxide interface engineering.