o, oh

Except as an infrequent variant of oh, o is used in direct address, is always capitalized, and is not followed by any mark of punctuation: "O Susan, please come here." "O dear! " "O God in Heaven." Oh is an interjection, may be followed by a comma or an exclamation point, and follows the usual rules of punctuation: "Oh, what a shame." "Oh! what is that?" "But, oh how we loved that woman! "