In general, to find a symbol, open the "Insert Symbol" pane (at the
far right end of the Insert tab, the capital Omega) and look in the
dropdown list at the upper right for the Unicode range that probably
contains it.
Something labeled "Diameter sign," which looks like a Scandinavian
crossed o, is in the "Miscellaneous Technical" range, at position
2300. If you simply want to insert that symbol in your text, type
"2300" and then Alt-x (hold down the Alt key and type x).
(That works for any Unicode character at all, even if you don't have a
font installed that can display it.) I set the font to Cambria Math to
be sure I'd be able to see the character.
