The AUR has two Orcaslicer packages: orca-slicer and orca-slicer-bin. Which one to pick? And when launched, it doesn't display correctly because of GDK scaling. Here's how to install it properly.

Which Package

The version number of orca-slicer-bin is higher. orca-slicer builds on your system. orca-slicer-bin is the prebuilt AppImage version. When there's no reason for a local build, prefer orca-slicer-bin.

Scaling

Omarchy uses a GDK_SCALE of 2 by default. Most applications handle that neatly, but Orcaslicer doesn't. The windows are blown up and parts do not appear on the screen, which makes it unworkable. Fix it in the desktop file. Leave the installed desktop file alone and create your own version in ~/.local/share/applications/Orcaslicer.desktop:

[Desktop Entry]
Name=OrcaSlicer
Exec=env GDK_SCALE=1 /opt/orca-slicer/bin/orca-slicer %F
Icon=OrcaSlicer
Type=Application
Categories=Utility;
MimeType=model/stl;application/vnd.ms-3mfdocument;application/prs.wavefront-obj;application/x-amf;x-scheme-handler/orcaslicer;

That's it.