When it comes to monitoring standby databases from OEM configure the monitoring account with SYS using SYSDBA.
Granting SYSDBA privileges to user dbsnmp is not a best practice. You should either use user sys or another user with SYSDBA privileges for monitoring standby databases.
Note that when you set up monitoring settings for a standby database in the Configure Database Instance: Properties page, select "SYSDBA" from the Role drop-down list.
This is an expected behaviour as there is more checking done internally when a user other than 'sys' logs in as sysdba from the console.
Use the following note from metalink. This is a common issue. following this note will fix the issue,
Warning : User Dbsnmp Does Not Have Privileges To Monitor This Database (Doc ID 604389.1)
In my last installation I did the same and it worked so I'm not sure what, if anything, I did different now. Guess I'll use sys then.