One of the oddities of the English language is that an unspoken letter can wield tremendous power.
Such is the case of “Oh” versus “O.”
You see, “O” is used to show that someone is being spoken to directly. Today this is used almost exclusively in formal contexts.
“Oh,” on the other hand, is an interjection that can express anything from a topic change
Once upon a time, spelling anything in English was a largely arbitrary endeavor, and historical examples of a vocative “Oh” exist. However, until I invent a time machine, I’ll keep cringing when I see the two words confused.
This is especially true in venues of worship.
The polite way to invoke one’s deity uses “O.”
By contrast, the only justification for “Oh my God” in a praise song involves using the first word as an interjection of dismay.
“O God!” = appropriate when calling upon one’s deity.
“Oh, God!” = appropriate when 1. Complaining to one’s deity about said praise song 2.Taking the name of one’s deity in vain*
*which is actually never appropriate. Shame on you.