It is not recommended to use both Oracle compression and Commvault compression. This will only chew up CPU cycles and not gain any additional space savings. Plus, enabling Oracle compression will also impact DDB in a negative way. Where as Commvault compression is designed to work with Commvault DDB (See the responses from Anthony & Winston) which will yield the best possible space savings.
So the recommended best practice for Oracle = CV Compression ON + CV DDB. Also it is recommended to set BFS (In subclient settings) to between 4 & 8 which will yield in better deduplication & performance. Also keep in mind that archive log backups automatically disable deduplication (since they are all unique) and achieve better results with only compression.
Hope this helps...
Ernst F. Graeler
Senior Engineer III