Can I load georeferenced raster map data onto the Garmin Oregon?
No. Although the 400i and 400c use satellite imagery as part of their basemap and GB Discoverer maps are rasterized maps there is currently no way to take raster map data, georeference it and load it into the Oregon. There are several third party tools which may allow you to load raster map data onto the Oregon. Moagu supports the 60/76csx handhelds but does not currently work on the Oregon. Mapwel also seems to support the Oregon.