The same way you do in Azure today. Each database resource created by Oracle Database Service for Azure gets an Azure connection string you can use to connect to the database from any Azure application.
During onboarding, Oracle Database Service for Azure creates network connections between the cloud environments.
During database provisioning, Oracle Database Service for Azure defines the DNS entries and connection strings needed to access the resource from Azure.
Azure developers (and applications) don't need to know anything about Oracle Database Service for Azure-all they need is the connection string. Oracle publishes the connection string on the custom dashboard it creates for the database in Azure, so developers don't have to leave the Azure portal to get what they need to access the database from their applications.
Hence, the CORRECT ANSWER is "By connecting to the Oracle databases using connection strings"