Oxygen vacancies (V(O)) have profound effects on the physical and chemical performance of devices based on oxide materials. This is particularly true in the case of oxide-based resistive random access memories, in which memory switching operation under an external electrical stimulus is closely associated with the migration and ordering of the oxygen vacancies in the oxide material. In this paper, we report on a reliable approach to in situ control of the oxygen vacancies in TiOx films. Our strategy for tight control of the oxygen vacancy is based on the utilization of plasma-enhanced atomic layer deposition of titanium oxide under precisely regulated decomposition of the precursor molecules (titanium (IV) tetraisopropoxide, Ti[OCH(CH₃)₂]₄) by plasma-activated reactant mixture (N₂+O₂). From the various spectroscopic and microstructural analyses by using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, confocal Raman spectroscopy, and spectroscopic ellipsometry, we found that the precursor decomposition power (R(F)) of plasma-activated reactant mixture determines not only the oxygen vacancy concentration but also the crystallinity of the resulting TiO(x) film: nanocrystalline anatase TiO(x) with fewer oxygen vacancies under high R(F), while amorphous TiOx with more oxygen vacancies under low RF. Enabled by our controlling capability over the oxygen vacancy concentration, we were able to thoroughly elucidate the effect of oxygen vacancies on the resistive switching behavior of TiO(x)-based memory capacitors (Pt/TiO(x)/Pt). The electrical conduction behavior at the high resistance state could be explained within the framework of the trap-controlled space-charge-limited conduction with two characteristic transition voltages. One is the voltage (V(SCL)) for the transition from Ohmic conduction to space-charge-limited conduction, and the other is the voltage (V(TFL)) for transition from space-charge-limited conduction to trap-filled-limited conduction. In this work, we have disclosed for the first time the dependence of these two characteristic transition voltages (i.e., V(SCL) and V(TFL)) on the oxygen vacancy concentration.