| underline | <CTL CMD U> | ||
| overline | <CTL CMD O> | ||
| underbrace | <CTL OPT CMD U> | ||
| overbrace | <CTL OPT CMD O> | ||
| overrightarrow | |||
| overleftarrow | |||
| stackrel |
The Grouping submenu has commands for underlining and overlining, and related commands that produce underbraces and overbraces. There's also a command called stackrel used to construct compound operators via vertical stacking (see p. 50 of the LaTeX book for more information).