Using classic Outlook for Windows in business environments
You have to first turn off OneDrive on your device for it to stop saving everything. Then you have to go to OneDrive online. Sign in and just start deleting. If there is anything you want, save it to your device (laptop, desktop, etc.). You have to also clean out your email too. Go through your folders and get rid of everything you don't need. Empty the junk and trash folders too. Hope this helps.