A small word but can be a heavily loaded word.
When you receive bad news.
When you receive surprising information.
When you receive information and want to ackwnowledge receipt of said information.
When you don’t know what else to say.
An interjection in dialogue.
Synonyoms for ‘oh’: ahem, alas, amen, boo, er, hello, hooray, hurrah, shucks, whoopee, wow, ah, egad, golly, hey, huh, jeepers, oops, ouch, phew, phooey, pooh, psst, rah, ugh, umoh. Via dictionary.com