This study found that with the help of O2 nanobubble pre-oxidation treatment effective arsenic removal was successfully achieved and is expected to be applicable on an industrial scale. The main research findings to remove arsenic are as follows. The oxidation of As(III) by blowing out O2 nanobubbles, O2 millimeter-sized bubbles, and air nanobubbles was studied under the condition of As(V) equilibrium at pH 1of the Pourbaix diagram. At pH 1, only O2 nanobubbles were able to oxidize As(III) to As(V). At the same time, the oxidation rate of As(III) was about 20% in the presence of air nanobubbles and 0% in the presence of O2 millimeter-sized bubbles. According to the extended DLVO theory, O2 nanobubbles are unstable at acidic pH. Nanobubbles grow and break, and then OH is produced. Below pH 3, H3AsO3 reacts with OH and converts to H3AsO4 while As(III) is oxidized to As(V). Ferric hydroxide co-precipitation with arsenic was effective to remove arsenic ions at more than 20Fe/As mass ratio and pH higher than 4. The As(V) removal rate was higher than As(III) at acidic pH because anionic HAsO42− ion could be adsorbed onto positively charged ferric hydroxide. In the artificial mine wastewater treatment, the sedimentation height of coprecipitated sludge was reduced by O2 nanobubble utilization due to O2 and Fe(OH)3 hetero-coagulation.