root sqlplus / as sysdba ORA-12546: TNS:permission denied nt main err code: 516

One of the causes of such error I would like to describe below. 

It might sound crazy, but there is software of some vendors, and such software requires logon to an Oracle instance as sysdba from root OS account. 

I stuck into the error :

ERROR:
ORA-12546: TNS:permission denied

Addind root to group dba didn't help to resolve the ORA-12546, but adding root to oinstall did :). Actually root must be the member of group dba to avoid the

ERROR:
ORA-01017: invalid username/password; logon denied

You haven't to add root to these groups, you can simply use newgrp command (newgrp [-] oinstall , then newgrp [-] dba). After such 'manipulations' you will be able to logon as sysdba.

Good Luck !