600: Six hundred 601: Six “oh” one 602: Six oh two 603: Six oh three 610: Six (hundred) ten Informally (spoken US English) you may hear “o” in pronouncing 601-609. We cannot use it for 600. In similar fashion, we can say 1900 as "nineteen hundred" and 2021 as “twenty twenty-one” but we cannot say 2000 as “*twenty hundred” — we say “two thousand”.

'Oh' is zero certainly but it's more said in phone numbers, hotel rooms, flights. Not amounts. The only context in which someone might say 'six oh' in a price is after saying 'sixty' to clarify that it's 60 and not 16. However, 'six oh oh' for 600 would be strange because that can't really be misheard as something else. So, basically, I don't know 😄

You could say six-oh-oh maybe but it sounds odd to me.

Perhaps you heard six o’clock? Meaning 6pm or 6am.

As far as I know ‘oh’ = O = 0 = zero (British English)