QOD #1 — Solution

Solution

o, t, t, f, f , s , s , e

The letters are the first letters of the following sequence:
One, Two, Three, Four, Five, Six, Seven, Eight

Comments

I rewrote the solution slightly
One SSPer, frustrated with this puzzle or just too lazy to think about it, wrote on the posted question as follows:
o, t, t, f, Q , Z , X , Y
All I can say is that it was an interesting choice and ordering of letters...