Magnetism in atomically thin functional materials can be an important phenomenon for exploring two-dimensional magneto-optics. Magneto-optical experimental data have revealed significant Kerr signals in insulator thin films. Here, the magneto-optical Kerr effect of oxygen functionalized and doped hexagonal boron nitride (hBN) has been investigated by performing first-principles calculations. We calculated Kerr angle and Kerr ellipticity for functionalized hBN as an attention-drawn material. Moreover, increasing of oxygen doping percentage leads to the introduction of surface plasmon to hBN. Our findings show that the functionalized hBN can tolerate high-temperature conditions, keeping oxygen atoms bridge-bonded. These giant opto/magnetic responses of insulating 2D materials provide a platform for the potential designing of magneto-optical devices.