OK, for overline try [o]Overline[/o] which should give
overlined text like this between the tags.
This is the letter O not the number zero.
Having discovered this
https://html-shark.com/HTML/Overline.htm I have added 'o' as a custom BB code but not tested it thoroughly. In particular I have not tested it in conjunction with other tags/styling. If it works OK I will add a button for it in due course.