> But OpenJDK is Oracle's JDK
Not quite. There is some code that isn't the same due to license agreements with third parties.
Some JavaFX components are an example thereof.
There are also some enterprise features there are only available in the Oracle JVM version, in terms of security and monitoring.
Oracle's no golden example as a leader in open source (let alone libre software), but they demonstrate the probable trajectory for the Microsoft projects at this point: a healthy, cohesive community of developers (developers!) using the technology in an ever increasing number of projects and platforms.