Opening the database steps

In NOMOUNT stage

In nomount stage, as your instance is started by searching the parameter file spfile<SID>.ora, you should be able to see the background processes associated with the instance and  SGA size created.


In MOUNT stage

In mount stage you can do the following
   1. Rename data files,
   2. Enable/disable archive mode of database
   3. perform media recovery
   4. View v$log,v$logfile,v$datafile

In OPEN stage

In open mode, Oracle will check the availability of data files and online redo log files for consistency and instance recovery will happen if DB not shutdown properly