The letter O on my MacBook no longer works I no longer get any functionality when typing the letter O of my mid-2015 MacBook Pro.
How can I troubleshoot this?
If the problem is due to debris under the key cap you can use compressed air to blow under the O keycap.  If that doesn't fix it then you'll need to replace the entire keyboard.  There are guides and instructions on ifixit.com.  It would mean completely removing everything to get to the keyboard underneath. The keyboard itself is low cost. The time it will take to disassemble and reassemble without breaking anything and putting all the tiny screws back in the correct holes makes it difficult enough that you may wish to have a professional repair it.