DigitalVolcano Software Support

How do I delete duplicate files that are in OneDrive using Duplicate Cleaner 5?

You can just scan the OneDrive folder which is synced on your PC - the changes should be reflected in the cloud if the OneDrive app is running. Note that if you are comparing file contents the file files will be downloaded locally to your PC, taking up space.

Note that if you are comparing file contents the file files will be downloaded locally to your PC, taking up space.

Yes, I don't see that as a big problem. There are options to save the OneDrive info on disk or keep in the cloud. If you keep it in the cloud, I find that DC5 will time out on larger files. Also, one must be selective as to which files are kept on you computer because the OneDrive can have 1TB of memory.
I try to 'clean' only one OneDrive directory at a time . Usually only the Documents directory.