There are several ways you can insert the letter "o" into a document on a Windows machine if the key on your keyboard is not working correctly.
First: open a document or a website that contains the letter "o". Use your mouse to highlight the letter "o". Press [ALT]+[e] and then [c] to copy the letter to your clipboard. Use [ALT]+[e] and then [p] when you need to paste the letter into your document from the clipboard.
You can also use the character map. Click the start button and from programs select "system tools" and then "character map." Choose your font from the character map window. Click the letter "o", the [select] the [copy] to place the character in your clipboard and then paste it as needed into your document.