The method based on oxidation-coagulation-adsorption at optimized pH (OCOP) using NaHCO3, KMnO4 and FeCl3 as pH conditioner, oxidant and coagulant, respectively, is an efficient and low-cost method for removal of arsenic from groundwater that has been gaining popularity in India. In a recent modification of OCOP, coexisting ferrous iron of water was utilized in order to lower the dose of FeCl3 and hence to lower the cost of the treatment of water containing both arsenic and iron. Here we present how the OCOP method can be effectively and economically used for simultaneous removal of arsenic, iron and manganese ions from water containing all three contaminants. For this we have optimized the required doses of NaHCO3, KMnO4 and FeCl3 by response surface methodology and used the optimized doses in a field trial with 11 domestic units of 20 L capacity in batch mode. Arsenic, iron and manganese were removed simultaneously by the present modification of the OCOP method to below 1 μg/L, 0.03 mg/L and 0.009 mg/L from initial concentrations of 100 μg/L, 1–8 mg/L and 0.5–5.0 mg/L, respectively, with a reduced cost.