How do i prevent executing os command in oracle 11g
There is no reasonable way how to do that. For example till 11gR1 SELinux was not certified to be used with Oracle database. Since 11gR2 this is at least not recommended.

Let's assume that you and Oracle DBA are adult, where is the problem then?
Oracle is not running with root privs. On the other hand Oracle requires many kernel tunables(sysctl) to be set according to Oracle's manuals, and also Oracle monitoring might require execution of some OS commands, so you should cooperate with DBAs anyway.