Grant DBA rights to specific user

$ sqlplus

SQL*Plus: Release 11.2.0.1.0 Production on Sat Jan 7 13:52:22 2017

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Enter user-name: sys as sysdba
Enter password:

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> grant dba to availabilityuser with admin option;

Grant succeeded.