Do all nouns that end in an ¨o¨ are masculine?

No

Explanation:

While it is usually convention that words, especially nouns, that end in "o" are considered masculine and words that end in "a" are feminine, it is not always the case.

Here is a counterexample:

hand - English

la mano - Spanish

As you can see, "hand" has a feminine article ("la") in Spanish, despite ending in "o".

Hope this helps!

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