To install OC, install the Media Manager 5 as normal. OC is available as a Subapplication of MM.
Next, download the following file:
Replace all references to "localhost", with the OC site reference. E.g. if OC/MM is installed at "mm.digizuite.com", then replace "localhost" with "mm.digizuite.com/oc".
If there are any labels that needs to be changed on the ribben, they are in the bottom of the XML file, and can replaced at your leasure.
Now that the XML file is ready, have the Office 365 administrator upload the file, following the documentation here: https://docs.microsoft.com/en-us/office/dev/add-ins/publish/centralized-deployment
Once that is done, Office needs to be connected to that installation.
This is done by opening an Office program, e.g. Word, going to File → Account, and logging in there, with an account that is part of the organization the plugin was distributed to.