Adam Porter <adam@alphapapa.net> writes:

> There is no built-in way to do that, and no way independent of
> org-export to get the numbers, AFAIK.
>
> Here's some ugly old code that shows outline numbering as overlays in an
> Org buffer.  It doesn't update automatically, so you have to run it
> again when the outline changes.  But it seems to work well.  It uses
> dash.el and let-alist, in case you don't have them loaded.

Please do disregard that monstrosity.  I rewrote it in a much simpler
way.  It seems fast on a reasonably large Org buffer.  It doesn't
account for org-indent-mode or org-bullets, but that wouldn't be too
hard to fix, I think.

No third-party packages are required.  See attached file, or
<https://github.com/alphapapa/unpackaged.el#outline-number-overlays>.