That’s not really the right approach on OCI, unfortunately: if you just flush the rules you also break a lot of their management plane.
You’d want to modify the /etc/iptables/rules.v4 and rules.v6 files to add any rules you want to load on boot (and, of course, if you just flush the rules without saving them, then it won’t persist and a reboot will break things, again).
It’s an arguable benefit: I’m a fan of having the security policies AND iptables sitting between me and doing something stupid, but I also spent most of the last decade dealing with literally thousands and thousands of compromised hosts that just whoopsie oopsed redis/jenkins/their database/a ftp service in a publicly accessible state, got hacked, then had the customer come crying to us asking why we didn’t keep them from blowing their foot off - which, basically, is what the OCI defaults do.