Basically, yes. However, saying at "9 sharp" implies that the person you are speaking to should not be late, not even a minute or two after 9 o'clock.

Using 9 o'clock means around 9:00 by minutes.. But 9 sharp means exactly at 9:00 no minute more/less So using -sharp- makes it more specific

"Sharp" means "exactly on time." "We plan to take the 9:14 train, and there isn't one after that until 3:20, so please be here at 9 o'clock sharp." Also: A synonym for "sharp" is "on the dot"--"Please be here at 9 o'clock on the dot." The adjective "punctual" means "exactly on time." "We need to have you here at 9 o'clock, so please be punctual."

Like Irene said, "at 9 sharp" adds the nuance that the clock MUST read "9:00". If the clock reads even "9:01", you are late. Or if it reads "8:55", then you are too early. When people say "at 9:00", people generally don't mean literally, so you could arrive at 8:55, 9:05 or something reasonable. 9:30 is not reasonable. Hahaha :)