Fe65Ni17P11.5C6.5 bulk metallic glasses (BMGs) containing a different amount of 92, 181 and 252at.ppm oxygen were prepared and their deformation behavior under compressive loading were studied. It was found that the increment of oxygen increased strength but significantly decreased plastic deformation capability. X-ray photoelectron spectrograph (XPS) valence-band spectrum confirmed that oxygen doping tends to transfer interatomic bonds from s-like to p-d hybrid bonds, leading to the lower compressive plasticity. Our findings indicate that room-temperature brittleness of Fe-based BMGs is closely related to the concentration of oxygen impurity and the preparation process has to be carefully controlled.