N x F S
O
Which is the odd one out?
My opinion is that each of the members of the quintet (MOQ) may be considered an outsider.
I‘ve read the current solutions specifying the obvious features of the fonts like minuscule, bold etc and therefore will add other remarks addressing the use of the MOQ.
N is the only letter defining (in math) a certain set of numbers - R,Q, and Z are not MOQ.
x is the only one (Chemistry) depicting a group of elements, called halogens, the other 4 in MOQ define elements.
F (music) is the only one denoting one of seven notes in diatonic scale ABCDEFG.
S uses dots only (...) In the International Morse Code.
O is the only letter used as a digit zero replacement :
agent double O seven, my phone is double two triple O seventeen .
It is also the only letter with infinite number of symmetry axes.
It is unique, occupying an odd place in the ABC, all other MOQ are placed on even locations.
...need I go on?
I believe I made my point.
Another approach
