We develop oxygen pumping model systems to actively and reversibly control oxygen non-stoichiometry in oxide thin films using solid oxide electrolytes. By electrochemically tuning the oxygen chemical potential, we enable operando investigation of oxygen vacancy formation, redox processes, and structural changes without altering temperature or gas atmosphere. Combined with Raman spectroscopy and electrochemical characterization, this approach provides direct insight into oxygen-driven degradation and redox instability in model oxides and solid-state battery cathode materials.