Oracle Autonomous AI Database allows both one-way TLS as well as mutual TLS (mTLS) for connection, the default being the later. Refer to QuickStart with Oracle AI Autonomous Database for successfully connecting to the Autonomous AI database.

DigiCert retired the Organizational Unit (OU) field for all public TLS/SSL certificates to comply with industry standards as of August 2022 per their announcement. This means that the public TLS/SSL certificates issued by DigiCert will no longer have an OU field. Refer to MOS note 2911553.1 for details.
To avoid disruption to applications connecting to Oracle Autonomous AI Database Serverless (ADB-S) during the server side certificate change while preserving security, you must use hostname-based matching (a.k.a Domain Name (DN) matching) of the server certificate (for TLS server-authentication).
The following versions of JDBC-Thin support hostname-based matching:
ADB-S connect strings contain hostname "...(HOST=xyz)..." which depends on the region. For example, in the Chicago region the hostname would be "adb.us-chicago-1.oraclecloud.com". If you have replaced the default hostname with an IP address or a custom hostname, then hostname based DN matching will fail.
The solution is to add a new entry to your /etc/hosts file using the original ADB-S domain suffix. Your connection string should then use this new name. For example, add an entry "localtunnel.adb.us-chicago-1.oraclecloud.com" that resolves to your custom IP address and use that name in the JDBC connection string.