To answer your question: no, there is no other plugin needed. The "Oracle Spatial GeoRaster" plugin is all you need. But then again, there are so many things that can go wrong ... A lot can depend on where you got the GDAL build from and how the Georaster plugin was built. As a separate dynamic library ? In the GDAL core ?
Then also you are using Windows which adds its own lot of complexities with all te mix up of 32 vs 64-bit kernel and libraries ...
You should be able to access the GDAL version you are using with QGIS from the command line, right ? Let's confirm that works fine all the way.
What happens when you do the following ? Can you see the GeoRaster plugin ?
$ gdalinfo --version
GDAL 2.0.1, released 2015/09/15
$ gdalinfo --formats | grep GeoRaster
GeoRaster -raster- (rw+s): Oracle Spatial GeoRaster
Also you may want to check that your rasters can be accessed, just to make sure the connectivity is all right all the way down to the tables in your database. For example (substitute with your own connection details):
$ gdalinfo georaster:scott/tiger@localhost:1521/orcl121,us_rasters,georaster
Driver: GeoRaster/Oracle Spatial GeoRaster
Files: none associated
Size is 512, 512
Coordinate System is `'
SUBDATASET_1_DESC=[4299x4299x3] CellDepth=8BIT_U SRID=26943
SUBDATASET_2_DESC=[4299x4299x3] CellDepth=8BIT_U SRID=26943
SUBDATASET_3_DESC=[4299x4299x3] CellDepth=8BIT_U SRID=26943
SUBDATASET_4_DESC=[4299x4299x3] CellDepth=8BIT_U SRID=26943
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 512.0)
Upper Right ( 512.0, 0.0)
Lower Right ( 512.0, 512.0)
Center ( 256.0, 256.0)
All this to confirm that your GDAL environment is setup correctly. The next step is to verify that QGIS uses that environment correctly too.
BTW, what is the exact error you see ? Is that some GDAL exception ? Or does it come from QGIS.
One possible debug technique is to enable GDAL debugging:
Do that in the terminal window you start QGIS from. You should then see all sorts of debugging messages coming out on stdout (= in your terminal window). Maybe that will tell you more ...
On windows do
Then when you run QGIS (in that window) you should see things like this (you also see this if you run the gdalinfo command line):
GDAL: Auto register /opt/gdal/gdal-2.0.1/lib/gdalplugins/gdal_JP2OpenJPEG.so using GDALRegister_JP2OpenJPEG.
GDAL: Auto register /opt/gdal/gdal-2.0.1/lib/gdalplugins/gdal_ECW_JP2ECW.so using GDALRegister_ECW_JP2ECW.
GDAL: Auto register /opt/gdal/gdal-2.0.1/lib/gdalplugins/gdal_GEOR.so using GDALRegister_GEOR.
GDAL: Auto register /opt/gdal/gdal-2.0.1/lib/gdalplugins/ogr_OCI.so using RegisterOGROCI.