Upon careful observation of the series one can find a difference in the position of letters in A through Z series as
2,3,5,7,10,15 ( B's position in A to Z series is 2 , C's position is 3 , E's position is 5 so on .... ) . And the skipped letters difference is 0,1,1,2,4 hence next number in derived number series must be 8. So final derived series will become 0,1,1,2,4,8 hence we have to skip 8 letters after O and needs to pick ninth letter that is X .