The dependence of the magnetic properties on oxygen concentration in epitaxial La ₂ Co Mn O ₆ thin films deposited on (100)-oriented Sr Ti O ₃ substrates has been investigated by varying the oxygen background pressure during growth using pulsed laser deposition. Two distinct ferromagnetic (FM) phases are revealed, and the relative fraction varies with the oxygen concentration. The existence of oxygen vacancies induces the local vibronic Mn ³⁺– O – Co ³⁺ superexchange interactions in direct competition with the static FM Mn ⁴⁺– O – Co ²⁺ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions.