But if all you're looking for is an easy way to distinguish between a zero and the letter O in a list of serial numbers or the like, either will do the job. This is ANSI character number 216, a variation on the capital O alphabetic character. The result will look pretty much like a slashed zero, except that it will be a bit wider in most fonts. If the difference in size bothers you, the other choice is to use the equation field to define an overstrike. To create the field, hit Ctrl-F9 to insert the two field characters, represented by boldface open and close curly braces.
Word leaves the cursor between the two markers.
The two characters to overstrike, of course, are the slash and the zero. Be careful not to leave any spaces between the right parenthesis and the right curly brace. If you do, the spaces will be added to the field as extra characters. After you've entered the text, while the cursor is still between the two curly braces, hit F9 to update the field. If you have Word set to the defaults, you'll see the field change to a slashed zero.