An atmospheric pressure He/O2 COST reference microplasma jet was used to study interactions of O atoms with four different organic micropollutants used as target compounds treated by plasma in water (atrazine, carbamazepine, 1,7‐α‐ethinylestradiol, and bisphenol A). Various ratios of O2 to He were used to achieve different exposures of reactive oxygen species to the plasma‐treated micropollutants, and their degradation efficiencies were determined. The degradation of all four micropollutants was merely associated with the variable exposure to the O atom, whereas the role of O3 was minimal. A comprehensive investigation was conducted on the degradation pathways and toxicity reduction assessment of atrazine after the treatment by plasma.