ORA-16649 with Observer

When observer is running it takes full control of your db status. ORA-16649 will be given if you  start primary database after it crashed. This is not an issue when the observer is running.

Check if the observer and fast_start failover is running. If you find it as running, just wait for few minutes based on how long and how active the new primary database.
The database will be put on to standby mode, it'll get reinstated and it'll start to roll forward.

SQL> startup
ORACLE instance started.

Total System Global Area 5016387584 bytes
Fixed Size     2934696 bytes
Variable Size  1107298392 bytes
Database Buffers  3892314112 bytes
Redo Buffers    13840384 bytes
Database mounted.
ORA-16649: possible failover to another database prevents this database from being opened