Identify the missing term in the following letter series:
K, O, S, ?

Let us check the numerical positions of the given letters in the English alphabet:
* $\text{K}=11$
* $\text{O}=15$
* $\text{S}=19$

The difference between the consecutive terms is constant:

$$15-11=+4$$

$$19-15=+4$$

Following the same logic, the next positional value will be:

$$19+4=23$$

The letter at the $23\text{rd}$ position is $\text{W}$.