The Oracle OJDBC JAR is a common dependency for anyone wanting to connect to an Oracle database from Java. Oracle has a Maven repository that you can add to download Oracle JDBC drivers and other Oracle JARs. However not all of Oracle’s JARs (notably including OJDBC6) are not available on their Maven repository.
Historically Java releases moved at a slow pace. Open source projects and companies could comfortably stay on the same version for several years. Recently the release cadence has quickened with Java’s new six-monthly release schedule. Starting with Java 9, a new version is released every six months. Now many applications and projects need to support a wide range of Java versions.
In the Java/Maven ecosystem there are many repositories that publish JARs publicly. This is an up-to-date list of the ones that you are likely to encounter.