Both OpenJDK and Oracle JDK are created and maintained currently by Oracle only.

OpenJDK and Oracle JDK are implementations of the same Java specification passed the TCK (Java Technology Certification Kit).

Most of the vendors of JDK are written on top of OpenJDK by doing a few tweaks to [mostly to replace licensed proprietary parts / replace with more high-performance items that only work on specific OS] components without breaking the TCK compatibility.

Many vendors implemented the Java specification and got TCK passed. For example, IBM J9, Azul Zulu, Azul Zing, and Oracle JDK.

Almost every existing JDK is derived from OpenJDK.

As suggested by many, licensing is a change between JDKs.

Starting with JDK 11 accessing the long time support Oracle JDK/Java SE will now require a commercial license. You should now pay attention to which JDK you're installing as Oracle JDK without subscription could stop working. source

Ref: List of Java virtual machines

You would find interesting this excerpt from an Oracle blog post:

Q: What is the difference between the source code found in the OpenJDK repository, and the code you use to build the Oracle JDK?

A: It is very close - our build process for Oracle JDK releases builds on OpenJDK 7 by adding just a couple of pieces, like the deployment code, which includes Oracle's implementation of the Java Plugin and Java WebStart, as well as some closed source third party components like a graphics rasterizer, some open source third party components, like Rhino, and a few bits and pieces here and there, like additional documentation or third party fonts. Moving forward, our intent is to open source all pieces of the Oracle JDK except those that we consider commercial features such as JRockit Mission Control (not yet available in Oracle JDK), and replace encumbered third party components with open source alternatives to achieve closer parity between the code bases.

As Oracle is responsible for creating both, it's clear that it will ensure that its clients will have good reasons for paying, and performance is the obvious means.

I believe that OpenJDK is interpreter-only JVM. This is easier to port as it has no architecture specific assembly code but, unfortunately, it's less performant.

OracleJDK I think takes advantage of the platform's floating point ABI (Soft Float on RP1 and Hard Float on RP2). It might also have some amount of platform-specific code to make it faster.

A JIT (just-in-time) compiler was once included in both, named Shark, but I have no knowledge if it is included in OpenJDK. Wikipedia OpenJDK does not mention JIT and I did find this old and troubling issue Remove Shark compiler. However, Wikipedia Java version history does include JIT.

If OracleJDK today includes a platform-specific JIT compiler, but OpenJDK does not, that might well explain the difference in performance,

Firstly, a JDK consists of a Java Runtime Environment (JRE) and various tools that help you to compile and debug Java code. The JRE is stand-alone and doesn't contain any compiling or debugging tools. So, if you're not into development and just want to run applications written in Java, then you can just install the JRE.

The Oracle JDK is maintained by Oracle (surprise) whereas OpenJDK is an open source implementation with contributions from the open Java community as well as Oracle. As far as I know, Oracle JDK’s build process builds from OpenJDK source code. So, since Oracle JDK gets its source from OpenJDK, there is no major technical difference between Oracle JDK and OpenJDK. My understanding is that all of the development and bug fixes happen in OpenJDK and then they are propagated to the Oracle JDK.

I'll quote this answer here:

In the days of Java 6, there used to be a big difference between the two of them, OpenJDK running slower and supporting less. However, nowadays, except for the java-web-plugin (which was rewritten by the community and named icedtea-web), they are exactly the same. Neither of the two versions is "wrong".

There are still people claiming OpenJDK is worse than Oracle's JDK, but it's become a myth.

There have been instances where people claim that they had issues while running OpenJDK and that got solved when switched over to Oracle JDK or vice versa. But these claims, generally, cannot be verified. Considering the fact that Oracle JDK gets its source from OpenJDK these claims will always be difficult to prove.

Also, on an unrelated note, you can create your own JDK provided you follow the Java language specifications. Many companies (like Twitter) have their own JDKs. Of course, there are certain processes (that I'm currently not aware of) you have to follow before you do that.