Oh | O


1. Use O when it has a strong connection with the words that follow (i.e. when there is no comma after it).

O mighty motor-mouthed one….

2. Use Oh when it is an independent exclamatory utterance (i.e. when there is a comma or exclamation mark after it).

Oh, what a bunch of twaddle!

Quotation -
"The present tendency is to restrict Oh to places where it has a certain independence and to prefer O where it is proclitic or leans forward upon what follows."
(Source: Fowler's Modern English Usage s.v. "O and Oh")

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