first an instance is created 

Instance is a combination of sga and background processes. This instance is instantiated by reading the initsid.ora.
If there is any errors u will the message ,error in initsid.ora or init parameters

Next the database is mounted.

the control file is read (which is identified by
the initsid.ora)
By this oracle does the integrity check. 
control is Most important file for oracle to function.
If there is any errors you will get an error stating in opening the control file or mounting the
database

Next the database is opened.

if all the three previous checks are OK, the oracle makes the datafiles and logfiles ( identified by the controlfile in previous step) available to public
   
                              

                                     --------
                                     | open datafiles and logfiles (open the database)
                                     
                                     |  
                              -------    
                              | open the controlfile (mount database)
                              
                              
                      --------
                    identify the initsid.ora (create instance)