Site-to-Site VPN provides a site-to-site IPSec connection between your on-premises network and your virtual cloud network (VCN). The IPSec protocol suite encrypts IP traffic before the packets are transferred from the source to the destination and decrypts the traffic when it arrives.

The Oracle Database Service multitenant application can:
Create and manage resources in the subscription (for example, the custom dashboard, Azure App Insights, and Azure Log Analytics workspaces OracleDB for Azure creates for each provisioned database).
Stream OCI Database metrics to Azure App Insights and events to Azure Log Analytics.
Configure network settings in Azure so Azure resources can access the database resources in OCI.
Submit events to Azure Event Grid.
More read: Preparation and Prerequisites for OracleDB for Azure