Thank you for this question.
The letters A, I and O do not have any whole word meanings. These letters stand for whole words in print and consequently stand for whole words in braille as well.
Whatever rules you would follow for the Letters/Words “A” and “I” would also be followed for the Letter/Word “O”.
In your example, the ”O” is being used as a word, so the letter sign is not needed.
Since the letter “O” is not used as a short form word or contraction, the only time the letter indicator would be needed is if the “O” was being referenced as an actual letter.
I hope this helps.