http://www.orafaq.com/wiki/Unix_FAQ
Make sure the entry for your database SID in the ORATAB file ends in a capital "Y". Eg:
# $ORACLE_SID:$ORACLE_HOME:[N|Y]
#
ORCL:/u01/app/oracle/product/8.0.5:Y
#
The scripts for starting and stopping databases are: $ORACLE_HOME/bin/dbstart and dbshut. SQL*Net (or Net8) is started with the lsnrctl command. Add the following entries to your /etc/rc2.d/S99dbstart (or equivalent - for example HP-UX uses /sbin/rc2.d/*) file:
su - oracle -c "/path/to/$ORACLE_HOME/bin/dbstart" # Start DB's
su - oracle -c "/path/to/$ORACLE_HOME/bin/lsnrctl start" # Start listener
