Dear Vaibhav Shukla
 
Electron affinity of an element is the energy released from the atom when an electron is added to its valence shell. because the energy is released, electron affinity is always a negative value.
 
Size of F atom is too small and its electrons are too close to each other that there is high inter electronic repulsion among the electrons of F.
Any upcoming electron into the atom feels a large amount of repulsion by the electronic cloud of F and thus its not so easy for the electron to enter into the atom. thus the electron gain enthalpy of F less than the expected value. in fact it is even less than the electron affinity of Cl atom
similarly..
Size of O atom is small and its electrons are too close to each other and there is high inter electronic repulsion among the electrons of O.
Any upcoming electron into the atom feels a large amount of repulsion by the electronic cloud of O and thus its not so easy for the electron to enter into the atom. thus the electron gain enthalpy of O  less than the electron affinity of S  atom whose size is greater then that of O atom.
 
Regards
Arun (askIITians forum expert)