I saw a couple of questions recently about database versions. One of the recent changes in Ops Center 12.3.1 was support for Oracle Database 220.127.116.11. If you have an embedded database, the database is automatically upgraded when you upgrade OC to 12.3.1.
The questions, though, were about what happens if you have a customer-managed database.
"If I'm upgrading to 12.3.1 with a customer-managed database, do I have to upgrade the DB to 18.104.22.168?"
Nope. Oracle Database 22.214.171.124.0 or 126.96.36.199.0 are still supported for the remote DB, so you don't have to upgrade.
"What about a fresh installation?"
The same is true for a new installation. You can take a look at the Cert Matrix to see what's supported for what.