The order of first electron affinity of O,S and Se is:
Electron affinity decreases down the group the group, but 'O' has small atomic size and 2p-orbital becomes very compact and already has 6 electrons , hence, there is a repulsive force among the already present and added electrons. Some of the energy evolved, due to addition of electron is used to reduce the repulsion. Hence, the E. 0A. Of O is less than S, so the order is S >O>Se.