If you run it with sudo (As shown below), it will include some system files that it woud not normally have access to scan. That is a more accurate representation of what's consuming your drive.
Another option is to use this terminal command
sudo du -d 1 -x -c -g /
I prefer to redirect it to a text file (this puts it in your Documents folder
sudo du -d 1 -x -c -g / > ~/Documents/du.txt
Like the sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper command, it will scan all directories, but produce a text file as opposed to showing the results in a window