Existing Peer to Peer (P2P) collaborative editing systems do not al low any user to undo any modification. However, in such systems, users are not aware of others' modifications, hence, they could obtain duplicate work, conflicting modifications or malicious contents. In this paper, we propose a new undo framework called “UNO: Undo as a New Operation” in the Operational Trans- formation approach which does not limit the scalability of P2P algorithms. As a proof of concept, we apply our framework to build a P2P collaborative editor with undo capabilities in which any user can undo any modification.