Electron affinity is positive, when:
A. $O$ changes to ${O^ - }$
B. ${O^ - }$ changes to ${O^{2 - }}$
C. $O$ changes to ${O^ + }$
D. Electron affinity is always negative

Hint: The electron affinity suggests the energy required, when any element releases or accepts an electron to jump from any higher orbit to lower or sometimes even higher than the previous one. It is an endothermic process. Also, the electron affinity is positive in this case.

Complete step by step answer:
According to Bohr's postulate, electrons revolve in a stationary orbit around a positively charged nucleus. Here, the stationary word is used as electrons reluctantly do not fall inside the orbit nearer to the nucleus as if they do so continuously the nuclear positive charge will attract them and if electrons fall into the nucleus then the atom will get destroyed within${10^{ - 8}}\sec $. This doesn't mean that the electrons can never jump from its orbit to another orbit whether higher or lower. They can do so until the external energy concerned with it. When an electron is added to an atom, a negative ion is formed and a change in energy occurs. This change in energy due to gaining an electron is what we call the electron affinity. Always remember that as an electron is negatively charged so, after gaining an electron, it becomes a negative ion.
$A + {e^ - } = {A^ - }$
 When ${O^ - }$ Changes to ${O^{2 - }}$, it is said to be an endothermic reaction because ${O^ - }$ repels the incoming electron because they bear the same characteristics in case of charge, hence, it needs some energy to accept the electron. Then, the second electron affinity of oxygen is generally high. The reason behind it is that the electron is forced into a small, very electron-dense space. So, electron affinity is positive in this case.

Note: If we want to add one more electron after the addition of one electron then the electrons in valence shells will repel the incoming electron. So, we have to provide some energy to add one more electron to ${M^ - }$ ion. Hence, the second electron affinity of an element results in the endothermic.