Hi Jason,
If your OneDrive storage is full from iPhone photos you’ve already backed up, turning off camera backup stops new uploads but doesn’t remove existing files. To free space, you need to delete the photos from OneDrive after confirming they’re safely stored elsewhere (like your computer or an external drive) to avoid data loss. You can do this via the OneDrive app or website by selecting the backed-up photos and deleting them. Once deleted, your OneDrive storage will be freed.