For commercial use,I would definetely go with Solaris. But as far as I know there is no ORACLE version for intel solaris. So, if you want to work on a PC, Linux is fine. Both SuSE and Read Hat will work with ORACLE.

The ORACLE sql*plus commands and most of the databse handling is the same for microsoft. However, since most companies run ORACLE on a unix based server, it's better to install it on a unix system and learn to work with it.