Hi @frob
Based on my research, for SharePoint, we can use Managed Metadata (SharePoint Term Store) to add "tags", but at current time, we can not add tag for OneDrive files.
Here is a similar thread "Tagging in Onedrive for Business", you can have a look.
Besides, in this thread "Add the ability to add metadata tags to files" from Microsoft Feedback forum, Microsoft Feedback Team said OneDrive can not add metadata tags to files, OneDrive developers are continuing to explore this as part of the future roadmap.
Hope the information is helpful.
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