It’s the 144th birthday of our country, and to celebrate, here’s some grammar!
The word “oh” is an interjection. One often uses it to express surprise. For example, “Oh! Hello there! I didn’t hear you come in!”
The word “O” is not the same word. This will be easier to understand if you know a language that has cases, like Latin or Greek.
In Greek, nouns, adjectives etc. can take one of five cases, although sometimes there’s ambiguity among them. The nominative case is used for the subject of a sentence. The genitive is used to express possession. The dative is used for indirect object. The accusative is used for direct objects. Finally (and most importantly for understanding the word “O”) the vocative case is used for directly addressing someone or something.
“O” is a particle for introducing a classically styled address in the vocative case. You sometimes see this in religious texts in English: “Praise the Lord, O my soul!” So in this case the author is directly addressing his soul and telling it to praise the Lord.
You might also use “O” if you’re trying to be pompous or overly formal: “O great registrar! Hear my plea and grant me special permission to take Philosophy 601.”
You should not use “O” to express surprise. Saying, “O hi there” would be incorrect. You should also not use “oh” as an address, which brings me to why this is important for all Canadians.
If you write “Oh Canada” in reference to our national anthem, you are actually getting the lyrics wrong. It looks like you were surprised by Canada or something. “Oh, Canada, it’s you! I … I didn’t expect to see you here!”
The two words are homophones, like “you” vs “ewe”—different words, different spellings, same pronunciation. It’s “O Canada,” not “Oh Canada,” and there is a difference in meaning between them.