Undo is a tool in OE-cake that lets you undo the last action you did. By using a tool with the Delete element to delete blank space, you can create a "quicksave" before running a long simulation.
Dragging something, disabling gravity, and pausing cannot be undone. You can undo the Clear operation and un-resize windows by immediately clicking undo. If you click Clear or resize the window, then click the canvas you will not be able to undo.