You can make OmniDiskSweeper calculate other accounts if you authenticate and launch it through the Terminal. In a terminal window, type :

sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper

and enter your admin password when asked for.

Of course, as OmniDiskSweeper is now launched as root, you should be careful with that "Delete" button...