correct order of  electron gain enthalpy is

• Electron gain enthalpy($∆\mathrm{H} =-\mathrm{ve}$)
• Higher Electron gain enthalpy means greater energy is released when electron is added to gaseous isolated atom consider only magnitude but not the sign.
• Electron gain enthalpy of Oxygen is abnormally low  due to the repulsions between added electrons and lone pairs present on it.
•  Energy released is very low

expected trend -  O > S > Se > Te > Po

Actual trend -  S (-200KJ/mole) > Se > Te > Po > O (-141KJ/mole)