that is, if O and Obar use what they know so far based on the infromation from their past light cones, they could not possibly have A or Abar plotted in their diagrams if we try to use our usual sense
But general relativity said, since there's a spacetime manifold where events are plotted, and every event can be an observer by relate them with lorentz transformations, then they are all there in the diagram when the observers plot the events in their rest frame