Structural distortions in the oxygen octahedral network in transition-metal oxides play crucial roles in yielding a broad spectrum of functional properties, and precise control of such distortions is a key for developing future oxide-based electronics. Here, it is shown that the displacement of apical oxygen atom shared between the octahedra at the heterointerface is a determining parameter for these distortions and consequently for control of structural and electronic phases of a strained oxide film. The present analysis by complementary annular dark-and bright-field imaging in aberration-corrected scanning transmission electron microscopy reveals that structural phase differences in strained monoclinic and tetragonal SrRuO3 films grown on GdScO3 substrates result from relaxation of the octahedral tilt, associated with changes in the in-plane displacement of the apical oxygen atom at the heterointerface. It is further demonstrated that octahedral distortions and magnetrotransport properties of the SrRuO3 films can be controlled by interface engineering of the oxygen displacement. This provides a further degree of freedom for manipulating structural and electronic properties in strained films, allowing the design of novel oxide-based heterostructures.