In the United States we don't use a 24 hour clock. We would say 7 o'clock. 7 o' clock p.m to be exact, though we would usually drop the p.m. as it is implied whether we are in the morning or afternoon.

"o'clock" only goes from 1 to 12. So the answer would be "7 o'clock." If you wanted to specify AM or PM, you would say "7 PM." If you want to use 19 you would answer "nineteen hundred hours," at which point a third of the people wouldn't understand you because they don't know what it means, a third of the people would wonder why you're using military time* and if there was something about you they didn't know, and a third of the people would start assuming that you're a very organized, precise person who cares a lot about clocks. *That's what we call a 24 hour clock. It is very much associated with the Armed Forces.

No, it does not sound correct. We don't use 24 hour clocks very often, but when we do, we never use o'clock with it. The most natural way to say that would be 'at seven o'clock', 'at seven p m', or 'at seven o'clock in the evening'. If you specifically wanted to use 24 hour notation, you could say 'at nineteen hundred'.

Chinese people will say it like at 19o'clock. Because we don't use a.m or p.m very often, we call it Chinglish.But I teach my students to say it like at 7o'clock p.m.