CP866 is known in macOS as "DOS (Russian)", and although it's not in the menu by default, if a file has correct metadata, BBEdit will transcode it.
Otherwise you can use the "Reopen Using Encoding" command, and choose the appropriate encoding from the submenu after first enabling it in the settings (as shown in the attached).
R.
--
Rich Siegel Bare Bones Software, Inc.
<
sie...@barebones.com> <
https://www.barebones.com/>
Someday I'll look back on all this and laugh... until they sedate me.