The Empty example project creates an empty project with a Java class path set up to assist in the use of OCL.
This assistance is not really necessary now that the editors are based on Xtext and now that an XMI representation is not automatically saved in a bin directory.
The OCL editors can bed used wherever Ecore editors can be used.
It is not necessary for a project to have a Java nature.
It is not necessary for a project to have an Xtext nature. If you add an Xtext nature, your OCL files will be built automatically when other files in the project or its dependencies change. This can significantly clutter the Problems View if you have problems with your OCL, and may significantly increase build times.