Biochar cation exchange capacity (CEC) is a key property central to better retain soil nutrients and reduce fertilizer runoff. This paper reports a breakthrough progress in improving biochar CEC value by a factor of nearly 10 through biochar surface oxygenation by ozonization. The CEC value of the untreated biochar was measured to be anywhere between 14 and 17 cmol/kg. A 90-minutes dry ozonization treatment resulted in an increased biochar CEC value of 109~152 cmol/kg. Simultaneously, the biochar ozonization process resulted in a reduction of biochar pH from 9.82 to as low as 3.07, indicating the formation of oxygen-functional groups including carboxylic acids on biochar surfaces. Using the techniques of X-ray photoelectron spectroscopy (XPS), the formation of oxygen-functional groups including carboxylic acids on biochar surfaces have been observed at a nanometer molecular scale upon biochar surface oxygenation through the ozonization treatment. The molar O:C ratio (0.31:1) on ozonized biochar surface as analyzed by XPS was indeed significantly higher than that (0.16:1) of the control biochar surface. Surprisingly, the molar O:C ratio from the Elemental Analysis data also showed an increase from the non-ozonized sample (0.077:1) to the dry-ozonized sample (0.193:1). Fourier-transform infrared (FTIR) spectroscopy analysis also showed an increase in the content of oxygen-functional groups in the form of carbonyl groups on biochar surfaces upon ozonization, which can also produce certain amount of oxygenated biochar molecular fragments that may be solubilized by liquid water, potentially leading to greater effects upon application of biochar in soil.