A Microsoft file hosting and synchronization service.
It sounds like you're hitting a common wall with OneDrive storage. When your account is over its limit, OneDrive often prevents "regular" deletions because it can't even process the move to the Recycle Bin.
Here is the most effective way to clear that space:
Use the OneDrive Web Portal: Don't delete from the Finder/Sync folder on your Mac. Sign in directly to OneDrive.com. Web deletions are more reliable when storage is full.
The "Permanent Delete" Shortcut: Instead of just clicking delete, select a file and press Shift + Command + Delete. This attempts to bypass the Recycle Bin entirely, which often solves the "out of space" error.
Check the "Second-Stage" Bin: OneDrive has two Recycle Bins. Go to your Recycle Bin, scroll to the bottom, and click "Second-stage recycle bin". If this is full, your "deleted" files won't actually free up any quota until you empty it.
Outlook Attachment Cleanup: Since Microsoft 365 shares storage, your email attachments often count against your OneDrive limit. Check your Storage Summary to see if large emails are the hidden culprit.
Give the Shift + Command + Delete trick a try on the web version first—it's usually the fastest way to "unstick" a full account!