La-Co co-substituted magnetoplumbite-type (M-type) strontium ferrite is typically used as a base material for high-performance hard ferrite magnets. Generally, the unquenched orbital moment of Co2+, substituted for Fe3+, is thought to enhance magnetic anisotropy and then coercivity. La3+ is substituted for Sr2+ to compensate for the charge unbalance associated with Co2+ substitution. Even if the synthesis starts with equal amounts of Co and La, the Co contents of the resulting samples are generally lower than the La contents because of the partial reduction of Fe3+ to Fe2+. We studied how the oxygen potential suppresses the formation of Fe2+ and expands the Co solubility range in this system. Polycrystalline samples, synthesized under oxygen partial pressures of 0.2, 1.0, and 387 atm, were characterized by powder X-ray diffraction, wavelength-dispersive X-ray analysis, and magnetization measurements. We reveal that the application of higher oxygen pressure dramatically expands the Co solubility range, resulting in the enhancement in magnetic anisotropy. We successfully synthesized Sr-free LaFe11CoO19 with high magnetic anisotropy using the hot isostatic pressure (HIP) technique.