Postgres has Oracle layer, available as a commercial variant by EnterpriseDB. I don't think MariaDB implementation can compete, but EDB Postgres is solid. Basically a native implementation of Oracle in Postgres.
Which one? Good question. The mainframe, as/400, and Windows versions are all on separate source code last I heard.
https://www.enterprisedb.com/news/enterprisedb-and-ibmr-coll...