Technically what is the main difference between Oracle JDK and OpenJDK?

OpenJDK is a reference model and open source, while Oracle JDK is an implementation of the OpenJDK and is not open source. Oracle JDK is more stable than OpenJDK.

OpenJDK is released under GPL v2 license whereas Oracle JDK is licensed under Oracle Binary Code License Agreement.

OpenJDK and Oracle JDK have almost the same code, but Oracle JDK has more classes and some bugs fixed.

So if you want to develop enterprise/commercial software I would suggest to go for Oracle JDK, as it is thoroughly tested and stable.

I have faced lot of problems with application crashes using OpenJDK, which are fixed just by switching to Oracle JDK

Technical differences are a consequence of the goal of each one (OpenJDK is meant to be the reference implementation, open to the community, while Oracle is meant to be a commercial one)

They both have "almost" the same code of the classes in the Java API; but the code for the virtual machine itself is actually different, and when it comes to libraries, OpenJDK tends to use open libraries while Oracle tends to use closed ones; for instance, the font library.