The O-class vowels make some variation of the O sound in English. They are:
אֹ
A holem is a long vowel and is pronounced as the o in role.
אוֹ
A holem-vav is also a long vowel and is pronounced the same as a holem. cf defective writing.
אָ
A qamats-hatuf is a short vowel and is pronounced as the o in odd.
אֳ
A hataf-qamats is a very short or reduced vowel and is pronounced as the o in odd.