Outflowing oxygen ions (O+) have been observed to be a persistent feature in the Martian tail. However, not much is known of the spatial distribution of oxygen ions, of the correlation between oxygen ions and solar wind protons (H+), and, especially, of the acceleration of oxygen ions in the tail. We present a test particle simulation study of the motion of O+ and H+ ions and compare the results to Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA) Phobos 2 particle measurements. We have studied the spatial distribution, velocity, density, and flux of oxygen ions in the nightside. In the simulation the oxygen ions were ionized from the Martian hot oxygen corona and the H+ ions were solar wind protons. We used an empirical flow model to derive the magnetic and electric field everywhere around the planet. The work is the first test particle simulation for Mars where a fully three-dimensional magnetotail configuration has been used. The model reproduces many observed plasma features. The solar wind protons flow fluid-like near the terminator despite their finite Larmor radii. The oxygen ions produce a plasmasheet-like layer near the cross-tail current sheet and empty magnetic lobes, and have north-to-south asymmetry in the magnetosheath and in the tail much as observed. The energy of oxygen ions in the tail is close to, but slightly less than, observed. The particle density and the particle flux of oxygen ions also agree quite well with the observations, suggesting that the total O+ outflow rate is ~2¿1025 s-1. Overall, the study suggest that most of the observed O+ outflow features can be understood by assuming that the ions are accelerated by the convective electric field associated with the flow of the solar wind protons. ¿ 1999 American Geophysical Union