Controlling ordered oxygen vacancies offers a promising approach to inducing emergent phenomena and novel functionalities in complex oxides. However, achieving precise geometric control over oxygen vacancy channels (OVCs) in brownmillerite (BM) manganites, whether horizontally or vertically aligned, remains a challenge. Here, we demonstrate the modulation of OVC orientations—horizontal and vertical—in coherently epitaxial BM-${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{2.5}$ thin films by varying the crystalline orientation of ${\mathrm{NdGaO}}_3$ substrates, which share the same orthorhombic symmetry. Importantly, horizontal OVCs yeild an atomically sharp heterointerface, while vertical OVCs form an interface layer to maintain oxygen connectivity across the interface. All these films can undergo reversible topotactic phase transitions to and from perovskite ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$, accompanied by a crossover between half-metallic ferromagnetism and insulating antiferromagnetism. Furthermore, the modulated topotactic phase transitions are attributed to reduced manganese valence states, a significant suppression of oxygen-manganese hybridization, and enhanced on-site Coulomb interactions. These findings provide new insights into the design of oxygen-vacancy-engineered electronics with tunable properties, opening pathways for potential applications in memory devices and sensors.

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• Magnetic phase transitions
• Film deposition
• First-principles calculations
• Resistivity measurements
• X-ray absorption spectroscopy
• X-ray diffraction