Amorphous Co75.26-xFe4.74(BSi)(20+x) (0 < x) magnetic alloys were examined with Transmission Electron Microscopy (TEM) after magnetic field annealing. TEM analysis revealed that the crystallized Co layer under the surface oxide could be highly faulted with planar defects depending on the composition. Based on the electron diffraction, we have proposed a new form of stacking fault in which two oxygen atoms are substituted for FCC Co in the {111} planes to create randomly distributed clusters of oxygen atoms on the Co {111} planes. Such clusters would explain the absent {200} peak in the highly faulted composition of crystallized FCC Co and the streaks observed in the electron diffraction pattern of the material. The oxygen fault was also closely related to the magnitude of the induced magnetic anisotropy of the material, suggesting the Co-O bonds acting as the localized antiferromagnetic region.