What's the electronic configuration of Nickel and Oxygen in RNiO3 (R=Rare-Earth)?
What's the electronic configuration of Nickel and Oxygen in RNiO3
  (R=Rare-Earth)?


As an example, let's look at $\ce{LaNiO3}$.  Each oxygen has an oxidation state of -2, nickel has an oxidation state of +3 and lanthanum has an oxidation state of +3.  Overall the molecule is neutral.
$$\ce{La(+3)~ plus~ Ni(+3)~plus~ 3~oxygens (-6)~=~0}$$