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• Hybrid B1WC functional describes well the properties of and oxygen vacancy behavior in BaCoO 3. • Triclinic ideal BaCoO 3 is more stable than the cubic one. • Site symmetry approach within the supercell model allows one to obtain minimum energy states. We use the hybrid B1WC density functional calculations and the supercell model to analyze properties of BaCoO 3 (BCO) in the corner-sharing CoO 6 octahedra geometry. We compare the atomic and electronic structures for ideal and defective BCO containing a single oxygen vacancy (Vo). We find that the ideal BCO is better described by the triclinic space group (SG) P 1 supercell than the cubic SG Pm 3 ¯ m model. The site symmetry approach allows us to derive several Vo supercell models on the basis of SGs P 4 mm , Pmm 2 or Amm 2 from the aristotype cubic SG Pm 3 ¯ m. The low symmetry Pmm2 model suggests the 0.4 eV formation energy of Vo in BCO. Furthermore, stability of this result is confirmed by the calculation of Vo in the supercell without symmetry (SG P1). The ideal and defective BCO are characterized by the intermediate spin (IS) states of Co4+ and Co4+&Co3+ ions, respectively. Additional complexity is introduced by the smooth Co magnetic moment dependence on the Co–Vo distance. [ABSTRACT FROM AUTHOR]