Hosting a web site on Oracle cloud could be tricky, Even if everything is done right and configured the Security Lists in Virtual Cloud Network so the Ingress Rules contains port 80 for http and 443 for https and the Source CIDR is 0.0.0.0/0 and the Destination Port is 80 and after all that when you try to access the website using the public IP, then the reason could be the IP table insde ubuntu itself.
Go to the terminal, and edit the iptables
sudo nano /etc/iptables/rules.v4
Then add 2 lines (better to copy an existing one
-A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT
Then save and exit
Finally , run this command
sudo iptables-restore < /etc/iptables/rules.v4