I have Oracle 10g installed on RHEL4. I've given the commands as follows:
>conn sys / as sysdba
I get "ORA-01092. Oracle instance terminated. Disconnection forced." But if I give startup mount then the database is being mounted.
The STARTUP MOUNT option does not access the database datafiles. It is when the database moves from the MOUNT stage to the OPEN stage that the datafiles are accessed for the first time. Something is happening in the move from MOUNT to OPEN to cause your Oracle instance to crash. Unfortunately, I can only guess what that problem would be. To know for sure, you would have to consult your Alert Log. When in MOUNT mode, issue the following command:
SHOW PARAMETER background_dump_dest
This will return a directory. Look in that directory for a file titled alert_sid.log (where sid is your ORACLE_SID). The Alert Log will contain error messages indicating the exact reason why your instance won't open.