Undo/Redo has been recognized as an important fea- ture of collaborative editing systems. The undo mecha- nism allows any user to undo any edit operation at any time. Preserving consistency of shared data with the undo feature is a complex issue. Several algorithms ex- ist for managing Undo in collaborative editing, but they are still open issues concerning correctness or intrinsic limitations of these algorithms. Therefore, in this pa- per we present an novel undo approach in the context of Operational Transformation (OT) framework. We prove the correctness of our solution, we leverage some limitations of current Undo approaches and we indicate how it can be combined with existing OT integration algorithms to provide a complete usable system.