To enable the database service to start automatically on boot, add the following lines to /etc/systemd/system/oracle-rdbms.service file.
# /etc/systemd/system/oracle-rdbms.service
# Invoking Oracle scripts to start/shutdown Instances defined in /etc/oratab
# and starts Listener
[Unit]
Description=Oracle Database(s) and Listener
Requires=network.target
[Service]
Type=forking
Restart=no
ExecStart=/u01/app/oracle/product/12.2.0/dbhome_1/bin/dbstart /u01/app/oracle/product/12.2.0/dbhome_1
ExecStop=/u01/app/oracle/product/12.2.0/dbhome_1/bin/dbshut /u01/app/oracle/product/12.2.0/dbhome_1
User=oracle
[Install]
WantedBy=multi-user.target