The OPItems API is implemented since OPItems v1.12.0

Setup:
 - Download OPItems.jar and javadoc.zip from the latest GitHub release
 - IntelIJ is being used in this how2
 - Build Project with Gradle
 - Do not use Boostrappers / Loaders in your plugin -> Delete the classes and references in the .yml if they are generated
 - In the paper-plugin.yml:
    Add: load: POSTWORLD
    Add to dependencies:

    dependencies:
      server:
        OPItems:
          load: BEFORE
          required: true
          join-classpath: true

As seen in the dependencies, the classpaths will be joined. Therefore there must not be overlapping names.
    --> Do Not use the packes cel20.opitems.* ; Use own super-package, like name.project.*

 - Copy OPItems.jar and javadoc.zip to your project. root/libs/OPItems.jar is used in the example as the location of the file. The OPItems jar should always be the same used in the server if possible

 - Configure Gradle in file build.gradle.kts:
    - Add dependencies:
        Add: compileOnly(files("libs/OPItemsjar")) in dependencies {}

        Should then look something like:

        dependencies {
            paperweight.paperDevBundle("26.2.build.+")

            compileOnly(files("libs/OPItems.jar"))
        }

 - Force IntelIJ to reload the Gradle Project now:
        (Press the button with the 4 horizontal lines on the top left) -> File -> Invalid Caches -> Select all boxes -> Press invalidate and start -> Wait until restarted and the task of reimporting the gradle project is finished.

 - OPItems API can now be used: OPItemsAPI api = Bukkit.getServicesManager().load(OPItemsAPI.class);
    Example:
        OPItemsAPI api = Bukkit.getServicesManager().load(OPItemsAPI.class);
        Bukkit.broadcastMessage(api.getPluginAPI().getOPItemsVersion());

 - Press the cog on the top right -> Project Structure -> + -> Java -> Navigate to OPItems.jar and select
 - Dont close yet, Press + on the right side -> Navigate to javadoc.zip -> Select JavaDoc
 -> Documentation should now be shown

Important Notes:
 - The API can only be used after onEnable() was called
 - All OPItems code can be accessed through the API if deeper modifications are wished
 - OPItems is an old plugin and is still mostly programmed functionally, not object orientated, so most code can be accessed directly as static methods.
    -> Read the Javadoc for the scope of methods