Why do you need energy to add an electron to an O-atom?

you don't need energy.Energy is released.Unless the O atom is negatively charged. Suppose we have a spherical ball of charge.The surface energy of the charge at this point is say 0J,we can safely say so by holding the base value as the zero for our calculative ease.Then,we introduce a negative charge such as an electron of value -1.6 x 10^-19 C,thus once this charge is included into the sphere,the charge density increases,the net charge however,becomes a negative value,now energy is proportional to QV,thus the net energy becomes a negative one since V is a positive quanitity(such is a very good assumption since the negative charge is generally attracted by the nucleus of the atoms),thus the energy of the system decreases and thus some energy must have been lost.This energy,as quantum mechanics suggests,must be released in uniform packets.Thus this energy is released either as light quanta or as heat,since both of these energies are quantized! rarely,even sound is the form of emissive energy since sound energy is also a quantized form of energy! :D now this energy emitted decreases as the negative charge increases because it is difficult for a same charge to enter a similarly charged field,thus work continually becomes positive and potential constantly negative,this is the case for ions like N3-,X4- ions that are found in mass spectrometers and not naturally. Hope this helps ya :D