Random Riddle

The letter 'O'. It is not a letter in 'the alphabet'.
Hold your mouse over for the answer.
Where there are two, you see just one.
But if there are thirty, you will see none.
It comes in a month, but not in a year.
Horror has several, but it causes no fear.
It’s in Wisconsin, but not New Hampshire.
It’s in a response, but not in an answer.

HINT: It is not a letter in the alphabet.