Ois an electronegative element so when we add an electron to it, it gains that electron and forms O- anion. Thus energy is released in the formation and it is an exothermic process. But while another electron is added to this unstable O- anion then it faces repulsion due to the similar charges. So in order to overcome these repulsions due to similar charge external energy is required due to which this becomes an endothermic process.
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