Find the simplest pattern and insert the missing terms of the sequences. o, t, t, f, f, s, s, e, n, ...

The letters are the first letter of their corresponding numbers (one, two, three = o, t, t). This means that the next terms will be t, e, t, t, f, f, s, s, e, n, t and so on.

O is one

T is two

T is three

F is four

F is five

S is six

S is seven

E is eight

N is nine

The next two terms

T for ten and e for eleven