Ottfssen... which is th next letter?

Answer 1

n This appears to be the initial letters of (most of) the first nine numbers, spelled out: o = one; t = two; t = three; f = four or five; s = six; s = seven; e = eight; n = nine

the next letter should be 't' (for 'ten'), although the missing 'f' could be intentionally part of a pattern. If 'five' was left out, then 'ten' should be left out as well; the next letter in that case would be 'e', for 'eleven'.

Answer 2

T. The sequence is the first letter of the numbers one, two, three...