Once in OMS Console can't connect with SYSDBA Privileges

I have two databases. One is local and one is remote. The local one contains the repository.
The OMS runs locally and the OMS Console runs locally. In order to avoid confusion, I didn't set any
preferred credentials.
1) I enter the OMS Console with the OMS Console password. 2) I try to access the databases (with either SYS or SYSTEM and the initial standard passwords, but if I choose the SYSDBA option, I get the "insufficient privileges" error.
3)I can only access with the NORMAL or SYSOPER option, but that's not enougth for some DBA operations.

How do I fix this?
Best,

Luca

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