When you start OneSwarm in a terminal you get a lot of output messages.
When you start OneSwarm outside of a terminal in linux the X server saves these messages in the file .xsession-errors in your homedirectory (~/.xsession-errors).
In my case this file has gotten 2 GB big. I had the downloads on a different partition and this file was filling my home drive.
You can just delete this file now and then (if you don't need the messages given in there).
Another option would be to start OneSwarm like this:
OneSwarm 2> /dev/null
OneSwarm 2>&1 > /dev/null