Startup

To startup using sqlplus

sqlplus / nolog
connect username/password as sysdba

or
sqlplus /nolog
connect / as sysdba

Different scenarios

STARTUP - startup, open and mount.

STARUP NOMOUNT

STARTUP MOUNT

STARTUP RESTRICT - Only allows users with restriced session privilege to access the db.

STARTUP FORCE - aborts an instance and then start's it.

STARTUP OPEN RECOVER - If you know recovery is required you can do it with startup.

ALTER DATABASE OPEN - Make a mounted & closed database open.

ALTER DATABASE OPEN READ ONLY - This is usually used on standby databases.


nomount
Oracle reads the init.ora file, locates the control file, initialises the SGA and starts all background processes. (Database creation, recreating control file)
Mount
Oracle opens the control files and verifies that no other instance has mounted the database. (Alter database commands, media recovery, relocating data files)
Open
Oracle gets a lock on the data files, opens redo logs and crash recovery might be performed. Acquires one or more rollback segments or undo tablespaces.
Recover
This is used to force a media recovery.
Restrict
Performs a nomount, mount and open, leaving the database only accessable to users with RESTRICTED SESSION privilege.
Parallel Used when mounting the database in a parallel server configuration.