If using DD deduplication, should Oracle compression be enabled?

emcmagic,

Dynamox is correct:

https://support.emc.com/docu47194_Oracle-RMAN-Best-Practices-with-Data-Domain.pdf?language=en_US

Page 13:

Compression

RMAN provides two types of compression. All supported versions of RMAN can apply a lossless compression algorithm (BZIP2) to the data being written to the backupset. This results in less disk space being used at the cost of significantly greater CPU consumption during the backup operation than the alternate compression type. When using a Data Domain system as the target, RMAN lossless compression SHOULD NOT be used. Pre-compression of the backup streams will randomize the data patterns and reduce deduplication significantly.