Which is the correct ascending order of electron affinity?

The correct ascending order of electron affinity is $\mathit{O }<\mathit{ Se }<\mathit{ S}$. 
The electron affinity of an atom or molecules is defined as the amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form an anion. 

Electron attracting tendency is higher for non-metals and it increases across the period. 

Elements at the right side have high nuclear charge and hence more electron affinity.