Instructions are here on the Garmin Oregon Wiki.
If your Oregon freezes during the startup process (i.e. Loading Maps...), and you've just loaded new maps or new gpx files or custom icons then it could be the new files which are causing the problem. This procedure forces the unit into USB Mass Storage mode so that you can delete (or rename) these files. Follow these steps to recover:
1. Attach the Oregon to your computer via USB cable
2. The Oregon will power on and start to boot, immediately power the unit off while still connected
3. Power on the Oregon and continue to hold down the power button for 30 seconds
4. The Oregon should enter USB mass storage mode
5. From your computer remove or rename any gpx, map files (.img or .kmz) files that were recently added
6. Unplug the Oregon and power on to see if this remedies the problem
During step 3) you should only see the Garmin banner for 30 seconds followed immediately by the graphic that shows a computer connected to a GPS. If you see any other text such as your custom banner, banners for loading maps, waypoints, etc then you have not held down the power button long enough.
