It is known that surface terminations contribute significantly to diamond properties. As one of the most commonly studied types, oxygen-terminated diamond surface possesses a positive electron affinity (PEA) and hydrophilicity, making it suitable for electronic device fabrication and bioapplications. Various oxidation methods have been reported on diamond, but a systematic comparison is still lacking. Herein, we present a comparative study on the oxidation of microcrystalline diamond powder (DP) and polycrystalline diamond film (PCD) by wet chemical treatments, including various acid mixtures, as well as by dry processes, including O₂ plasma and UV ozone. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) results reveal that H₂SO₄/HNO₃ 5:1 at 360 °C demonstrates the best oxidation performance on DP and PCD compared to other wet chemical methods, while the PCD treated by O₂ plasma exhibits the highest oxygen content among all the treated samples. Besides, the concentration of carbon-oxygen groups on diamond surfaces is found largely determined by oxidation methods. These experimental results can be attributed to the corresponding oxidation mechanism of different treatments and agree well with theoretical simulations. Finally, we investigated the sp² bonded carbon concentration and surface roughness of oxidized PCD, suggesting the optimized diamond surface cleaning conditions.