A Microsoft file hosting and synchronization service.
What you’re seeing in OneDrive under Shared are items that someone else has shared with your account; they don’t live in your OneDrive storage, so you can’t truly “delete” them, only hide or report them. On the OneDrive website, go to Shared > Shared with you, right-click each suspicious file, and choose Report a concern. This sends it to Microsoft for review and abuse handling. There’s also an anonymous abuse form if you prefer not to sign in, described here: