Tunability of magnetic anisotropy in perovskite oxides, such as in SrRuO3, is commonly achieved by controlling the octahedral distortion through strain. Here, we demonstrate that differences in the oxygen vacancies at the heterointerface of SrMnO3/SrRuO3 can also strongly influence the magnetocrystalline anisotropy in SrRuO3 despite being fully strained by the underlying substrate. Modification of the spin–orbit coupling strength by altering the hybridization of Ru-4d and O-2p orbitals in SrRuO3 leads to a clear evolution of the magnetocrystalline anisotropy from multiaxial to strongly out-of-plane, as manifested in the magneto-transport studies. Our results provide an alternative design strategy for their incorporation in practical spintronic devices for memory and computing applications and operation by spin–transfer and spin–orbit torques.