Over 100 GBs of space on my HD is missing..

Question

Hi,
I have a 15 inch Macbook Pro mid-2010 model(320 GB) and I seem to be missing over 100 GBs of HD space. I was getting the error "Your startup disc space is almost full" yesterday so I checked and noticed I had only 2.6 GBs of HD space left. I went into storage and noticed that over 180 GBS of space was being taken up by other.
I went through and cleared out about 6 GBs of space from my downloads and ran Omni Disc Sweeper to figure out what the heck is going on. After running ODS, it came back as that I was only using 192.7 GBs of space but my computer was saying I was using over 310!
I figured out a way to clear out about 18 more gigs of space with the HD Cleaner app and now I have 26.6 GBs of space.
I ran a program called Disc Art to see if that will tell show me where this hidden GB space is and that came back as the same thing, Im only using 168.9 of my HD space but my computer is telling me I'm using 292.3 GBs of space.
I have googled everything and I have checked all of the suggestions, it is not the time machine snapshots(I don't even have time machine turned on now), the mail duplication error, giant log file, or any of those other common problems.
I am so confused,
Here is a screen shot of the storage
I think the majority of it is coming from the Other category, but I am lost right now.
Whatever it is, seems to be gradually decreasing my hard drive space. Hopefully someone has run across something like this before.
Thanks for any help.

Answer 1

ODS can't see the whole filesystem when you run it just by double-clicking; it only sees files that you have permission to read. To see everything, you have to run it as root.
Back up all data now.
If you have more than one user account, make sure you're logged in as an administrator. The administrator account is the one that was created automatically when you first set up the computer.
Install the app you downloaded in the Applications folder as usual. Quit it if it's running.
Triple-click anywhere in the line of text below on this page to select it, then copy the selected text to the Clipboard by pressing the key combination command-C:
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window by pressing command-V. You'll be prompted for your login password, which won't be displayed when you type it. Type carefully and then press return. You may get a one-time warning to be careful. If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator. Ignore any other messages that appear in the Terminal window.
The application window will open, eventually showing all files in all folders, sorted by size. It may take a few minutes for the app to finish scanning.
I don't recommend that you make a habit of doing this. Don't delete anything as root. If something needs to be deleted, make sure you know what it is and how it got there, and then delete it by other, safer, means. When in doubt, leave it alone or ask for guidance.
When you're done with the app, quit it and also quit Terminal.