There are many OpenJDK(-based) distributions out there, mostly built from the OpenJDK {8, 11} Update Releases sources. AdoptOpenJDK provides one of those distributions.
Most of the actual backport work is done by the OpenJDK participants, notably people from Red Hat (like me), SAP, Amazon, Azul and others. Oracle does not participate in OpenJDK 8u/11u directly, opting to maintain their separate tree, from which they build Oracle JDK 8u/11u available under the commercial license. There is also a security collaboration between many companies (OpenJDK Vulnerability Group), which drops its deliverables in relevant source trees.
At one point, I tried to draw this map to show the bird's eye view on the landscape: https://shipilev.net/jdk-updates/map/ -- you can find AdoptOpenJDK there.
What gets people confused is http://openjdk.java.net/ linking to Oracle distribution (either at jdk.java.net, or Oracle site). That might be seen as "OpenJDK providing the binaries", but it is actually not: it links to one of the distribution providers.