  The role of the oxygen degree of freedom in the cuprates' superconducting
planes is analyzed in detail. Structural and photoemission results are reviewed
to show that the most sparse description of the in-plane electronic states
requires explicit control of the oxygens. For metallic states, the relative
contributions of oxygen and copper vary along the Fermi surface, with the arc
metallicity dominantly oxygen-derived. For the magnetic responses, we find that
the observed incommensurability arises naturally if one keeps separate the
roles of the two sites. For the charge order in LBCO, we propose a scenario,
based on magnetic interactions in the plane. We stress the need for further
experimental investigations of the evolution of the intracell charge
distribution with doping, and for a better theoretical understanding of the
large particle-hole-symmetry breaking required for successful phenomenologies,
but difficult to reconcile with ab initio calculations.
