I'm on Windows. After git commit, I entered a message.
Then I tried Ctrl+O to save but it doesn't work (I guess I'm on nano, am I mistaken?).

I'm on Windows. After git commit, I entered a message.
Then I tried Ctrl+O to save but it doesn't work (I guess I'm on nano, am I mistaken?).

Nano was added as a choice in 2017 with Git For Windows, because of issue 1224: "Add nano as a choice of editor(s) for git-commit because vim increases what we have to tell to beginners" (!)
But the editor by default remains "vim".
In your case, since you have modified the comment, ESC followed by :x is enough.
If this is nano, commands are Ctrl + X , then Enter (maybe multiple times depending on your conf)