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Right click on one of the folders under the main OneDrive heading in Explorer. To keep on the PC the option is "Keep on this Device" to have in cloud only select "Free Up Space". If they are only stored in the cloud, there will be a little cloud icon beside a folder in the listing. If it is in both places the icon is a check mark within a circle, IIRC.
You may be able to do this globally (affects all sub-folders) by right-clicking on the Main One Drive item in Explorer.
I would check this out using a test folder before doing it with your real files to ensure it gives you what you want (and doesn't delete them in both places)!