Explanation :
The given series involves three steps :
1st Step : The first letters of each term in the series are in alphabetical order and beginning with O i.e. O, P, Q, R, S.
2nd Step : The second letters of each term in the series are in alphabetical order and beginning with T i.e. T, U, V, W, X.
3rd Step : The third letters of each term in the series are in alphabetical order and beginning with E i.e. E, F, G, H, I.
So, the missing term is SXI.