Unfortunately, I'm not familiar with the Pages equation editor and the line of text that Viking OSX mentions, or using the U+035e system that Tom mentions. However, I did find this process that works for me, at least in Times New Roman:
In the Character Viewer, find the "combining double macron," and save it to Favorites.
In the document, open the Character Viewer. Type the lower case "o" and then hit alt/option, then in Character Viewer, double-click the combining double-macron; that puts the combining double macron over the o and the next as-yet empty space.
Then type the second lower case o and it appears under the overhanging line of the combining double macron, putting the line over both letters.