To type diacritics (macrons etc.) in Microsoft Word:
Click on the Insert tab.
Click on Insert symbol, more symbols
Scroll down until you find the character that you want - like o with a line over it.
Then you can create a short cut so that you don't have to do all of this again.
Click on Shortcut key and assign a shortcut (like alt o).
Now when you want to type o with a macron, you can simply use alt o