Due to small 2p orbital of o there is a great inter electronic repulsion.the 3p orbital of sulphur suffers comparatively lesser inter electronic repulsion
						

							
oxygen shows exception beacuse the size of oxygen is very small and as a result having large electron repulsion , therefore tendency to gain electron is less . 
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							sulphur has more negative electron gain enthalpy than oxygen bcoz oxygen has relatively small size than sulphur and hence oxygen cannot accomodate more electons and to do so we need to provide extra energy and providing extra energy means more positive electron gain enthalpy thus answer is sulphur
						

							
the energy relesed by sulphur is more than that of oxygen to while accepting an electron
when energy is released we say that the enthalpy is negative
more the energy is released more negative is the electron gain enthalpy
oxygen is more electronegative than sulphur therefore the energy reqired to be released is lesser
						

							Oxygen does not have a d-orbital, but sulphur has a d-orbital. So, oxygen is not able to accommodate more electrons due to its small size showing more electron repulsion. Whereas sulphur has plenty of space in the d-orbital and has comparatively lesser electron repulsion.