How to add accents

Accents are applied from your keyboard. This is a system wide feature wherever you can type text. For example, if you need a acute accent, Option-e will start the sequence, then whatever vowel it is applied to is typed immediately after: ó [option-e, o]. All of the combination sequences use the Option key.

This will get you started : http://osxdaily.com/2017/03/22/type-accents-mac-easy/

In the meantime, if you're in a pinch, at a type character cursor, type Control-Command-Spacebar. That will pop up the Character Viewer. You can scroll down the character list and find what you need. Double click on the character to insert it at the cursor. You can access the entire Unicode range of characters (dingbats, emoji, etc...) this way.

☞◉ [Some characters are *not* compatible with 3D Text, such as Emoji.]

…How can i add an accent to "o" in final cut pro X? …

under Mojave, simply press and hold the O key:

after a beat, a window pops up with all available accents for that letter.

Select by mouse or press 1-8

Keep in mind not every font, esp 'free' ones, offers accents.