How do I stop OneDrive using so much space on my Mac?
Short answer: Right-click files in your OneDrive folder and choose Free up space or Remove Download; they stay in OneDrive and download when opened. Storage Studio can't free another service's downloads, because macOS doesn't allow it.
Make files online-only in the OneDrive app or Finder. Right-click files or folders in your OneDrive folder and choose Free up space (or Finder's Remove Download); they stay in OneDrive and download again when opened.
Storage Studio can't do this for OneDrive itself, because macOS only lets an app free the downloads of its own cloud service. What Storage Studio can do is show you how much space your OneDrive folder is taking and move content you no longer need in OneDrive to your external drive.