Overlining is comparatively straightforward in Apache OpenOffice and LibreOffice. But, for Microsoft Word users, overlining still isn’t a default formatting feature, as is its companion underlining. Sometimes inserting a symbol or special character will work if you can find one that matches the overlined character you need. In other cases, Word’s cache of symbols and special characters simply isn’t large enough to cover everything (e.g., when discussing nomina sacra). Sometimes, creating a character image might work, but inserting an image can create issues with text flow and line spacing.
Another option Word users have is to use field codes or the formula editor to insert the term that involves the overlining. In this case, vertical line spacing seems like it isn’t easily disturbed, but you may notice a bit of extra horizontal spacing between what’s inserted via the field code or formula editor and the rest of the surrounding text. Having overlining as a built-in text format would still be preferable, but using this work-around seems to yield pretty reasonable results.
(Update: HT to Andrew Kinsey via Facebook for correctly noting that I hadn’t previously simply collapsed the field code option into the use of the formula editor. The two methods achieve the same results but do deserve to be noted separately since one might be more convenient than the other.)