What all to start for oracle database server
If you have the entries for your instance added to /etc/oratab (that should be "orcl:/your/oracle/home:Y" ) and you did run the root.sh in the end of that installation and the root.sh placed the environment scripts in /usr/locla/bin (the default), you can issue:


ORACLE_SID=orcl
ORAENV_ASK=NO
. /usr/local/bin/oraenv
unset ORAENV_ASK
$ORACLE_HOME/bin/dbstart 


What oraenv does is mainly setting environment variables like ORACLE_HOME, ORACLE_BASE and PATH. PATH is made up of $ORACLE_HOME/bin. You could add the environment setup to your .profile and of course, you can start the database manually using sqlplus.

The dbstart script starts all databases that are registered in the oratab file, that have a 'Y' in the third column.

Manually starting, after the environment has been setup for a specific instance as shown above:

sqlplus / as sysdba
startup


If you want to switch to a different database just issue . oraenv (if /usr/local/bin is in the PATH) and give it the ORACLE_SID of the database that you want to manage.

Most databases will be accessed using a client from an other machine/VM. In that case you also might want to start the listener: lsnrctl start