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We report structural properties and diffusivity of liquid Fe0.96O0.04 over a density range of 5.421-11.620 g cm(-3), corresponding to pressures of 0-330 GPa and 2200-5500 K, using first-principles molecular dynamics. We predict a change in compression mechanism near 8 g cm(-3). At lower density, Fe and O coordinations increase from similar to 10 to similar to 13 and similar to 3 to similar to 6, respectively, the average Fe-O distance increases from similar to 1.81 similar to to similar to 1.88 angstrom, and the Fe-Fe distance remains essentially constant. Calculated oxygen diffusivities, DO, remain constant over the corresponding pressure range, consistent with experiments. For larger densities, interatomic distances and diffusion rates for both species decrease monotonically. Oxygen coordination reaches a maximum of similar to 8.5 at similar to 9.4 g cm(-3), indicating a local B2 packing for Fe around O under conditions of the Earth's core. The pressure-induced structural evolution provides an explanation for a previously reported change in pressure dependence of oxygen solubility in liquid iron.