Riddle: Can you solve it?

What are the next three letters in this combination? OTTFFSS 

This super hard question asks you to determine the relationship between the letters. They seem random: a vowel, two consonants from the tail of the alphabet, two from the front, then two more. What could they mean? What’s the pattern? In this case, think of common strings of words to get you closer. But it still seems almost impossible to hit on the solution.