If you've already downloaded OpenDisc but would still like to donate your bandwidth to keeping the torrent healthy, simply follow these instructions:
1) Download the .torrent file
2) Open it in your favourite torrent program
3) Once the program has added the torrent and made temporary files on your hard drive, close the torrent program.
4) Find the temporary file(s) on your hard drive and replace them with the completed OpenDisc .iso that you previously downloaded.
5) Reopen the torrent program, right click on the OpenDisc torrent and select "Recheck". This step might vary for some clients, from the wording of "Recheck" to it automatically checking each torrent on load.
After following that you should have an instantly complete torrent, ready to share. :)
Thanks again for donating your bandwidth to keeping the torrent healthy for new users!