Negative charge transfer ABO3 oxides may undergo electronic metal-insulator transitions (MIT) concomitant with a dilation and contraction of nearly rigid octahedra. On both sides of the MIT are in-phase or out-of-phase (or both) rotations of adjacent octahedra that buckle the B-O-B bond angle away from 180• . Using density functional theory with the PBEsol+U approach, we describe a novel octahedral engineering avenue to control the B 3d and O 2p orbital polarization through enhancement of the BO6 rotation "sense" rather than solely through conventional changes to the B-O bond lengths, i.e. crystal field distortions. Using CaFeO3 as a prototypical material, we show the flavor of the octahedral rotation pattern when combined with strain-rotation coupling and thin film engineering strategies offers a promising avenue to fine tune orbital polarizations near electronic phase boundaries. Transition metal oxide (TMO) perovskites are known to be strongly correlated materials, 1 whose properties are controlled by a complex interplay between geometric and electronic degrees of freedom. These are determined by considering the relative magnitude between various energy scales and interactions: the energy difference of the transition metal (M ) d orbitals and the oxygen p states, referred to as the charge transfer energy, and the strength of the on-site Hubbard U interaction of the d electrons. The charge transfer energy is more important in TMO where low-energy excitations are of p−d-type, whereas the Coulombic interaction, which localizes the electrons on the M -site, produces the insulating state in Mott-Hubbard systems.2 The properties of correlated electrons of TMO are controlled in part by the relative occupancy of the different transition-metal d orbitals.3 The relative d orbital occupancy is largely determined by the crystal field experienced by the transition-metal cation; this electrostatic field is the result of the 2p electronic density of the coordinating oxygen ligands. The latter, in turn, is directed by the extended geometric arrangement of the nearest neighboring oxygen atoms.Orbital occupancy can be tuned, for example, through chemical substitution, 4,5 epitaxial strain, 6,7 or by superlattice formation in thin films.8-10 Isovalent substitutions are important to charge transfer-type oxides, because rather than modifying the M -site electronic configuration, cations with different ionic sizes but the same formal valence renormalize the transfer interaction and the oneelectron bandwidths through changes in the crystal structure, i.e., interatomic bond angles and distances. The crucial distortion in ABO 3 perovskites is the buckling of the B-O-B bond angles, because the effective d electron transfer interaction between the neighboring transition metal sites is mediated by the angular overlap with the O 2p states.11 When the B-O-B bond angle deviates from the ideal value of 180• , the transfer interaction weakens and the bandwidth narrows. Such distortions to the inter-octahedral bond angles are typical in GdFeO...