Comment # 1 on bug 975387 from Wolfgang Bauer

I don't think adding that requirement is a good idea.
An application should *not* depend on the oxygen icon theme, that the icon
loader still falls back to oxygen is just a workaround for "broken"
applications, and actually a bug.

And kdenlive actually doesn't even require nor use oxygen icons when run
outside of a KDE session.

This seems to be a bug in the icon loader, apparently it seems to fail (in this
case at least) if the fallback theme (oxygen) is not found.
But this upstream commit (will be in KDE Frameworks 5.23.0) seems to fix it:
https://quickgit.kde.org/?p=kiconthemes.git&a=commit&h=ad1ead84ee2c3a14363afdb048b3579f3d01fa18