In this work we performed Rietveld refinement on neutron diffraction datasets and atomistic simulation to investigate the average and local structure of sodium nickel titanate (Na-2/3[Ni1/3Ti2/3]O-2), a two-layer ternary oxide with prismatic sodium sites (P2). It was found that sodium preferentially occupies the edge-sharing sites (Na-e) than the face-sharing site (Na-f) and preferentially moves in-plane than through plane. Two potential sets used for the atomistic simulation, Buckingham and Morse, both suggested that it is energetically favorable to have equal amount of Na and transition metal in each of the two layers. In addition to providing nuclear density maps similar to those form neutron diffraction experiments, the Morse potential model also indicates the possibility of 02 stacking caused by the sliding of transition metal oxide layer.