I've always been frustrated by only being able to have one copy of Oxygen running. I like to work on different projects in different workspaces, so when something large is building, I can just switch to another project for a bit. I discovered this:

https://www.oxygenxml.com/forum/topic14636.html

which solves the problem. Edit the oxygen.sh file in the Oxygen folder and add this:

-Dcom.oxygenxml.MultipleInstances=true\

in the set -- \ section. Then edit the .Desktop file that starts the Oxygen editor and comment out the existing line, which looks like this:

Exec="/home/mholmes/Oxygen XML Editor 23/oxygen23.1" %U

and add this one instead:

Exec="/home/mholmes/Oxygen XML Editor 23/oxygen.sh"

You have to omit the %U. Then you can start Oxygen as many times as you like. However, the last one to close will dictate the remembered settings.