It's from Javanese. Should be written as "Aja rame-rame!" but people tend to write it as "Ojo rame-rame".
'Aja' means "Don't... " (in Indonesian: "Jangan...")
'rame-rame' (in Indonesian: ramai-ramai) has 2 meanings, the first one 'to be noisy', the other one is 'to be crowded'. So depending on the context "Aja rame-rame"/"Jangan ramai-ramai" could mean "Don't be noisy/Shut up/Be quiet" or "Don't be crowded".
The first case obviously can be used if you want to tell people or children to be quiet. The second case can be used when you're visiting someone in hospital with your friend but you don't want the room to be crowded (take turns to go inside).
Ah, if you think your sentence would be ambiguous, in Indonesian you could say "Jangan berisik" (Don't be noisy) instead of "Jangan ramai-ramai".