The self-diffusivity of O has been measured in Na2Si4O9 liquid at 1625 and 1825 °C and pressures in the range 2.5–10 GPa using a 1200-ton multianvil apparatus. Diffusion couples were prepared from glass starting materials with one-half of the sample enriched in 18O and the other half containing the natural abundance (~0.2 wt%). Following exchange of O across this diffusion couple at high temperature and pressure, 18O concentration profiles were analyzed using an ion microprobe. O diffusivities were obtained by fitting the equation for diffusion between two semi-infinite bodies to the concentration profiles. At 1825 °C, O diffusivity increases continuously from 1 × 10−10 m2/s at 2.5 GPa to 4 × 10−10 m2/s at 10 GPa, yielding an activation volume of −3.3 ± 0.4 cm3/mol. This trend is consistent with the results of molecular dynamics calculations and may be related to an increase in fivefold-coordinated Si with increasing pressure. The viscosity of Na2Si4O9 liquid at high pressure is estimated from the O diffusivity data using the Eyring relationship, which has been shown to be applicable to polymerized silicate liquids up to at least 2GPa. Estimated viscosities at 1825 °C and high pressure are consistent with an extrapolation of viscosity data obtained at 1 bar by other techniques and suggest a viscosity decrease of ~0.8 log units from 1 bar to 10 GPa. The results also suggest that the activation energies for viscous flow and O diffusivity decrease significantly in the pressure range 1 bar to 4 GPa. Provided the validity of the Eyring relationship can be established, these results demonstrate that melt viscosities can be estimated from O-diffusion experiments up to at least 10 GPa. In contrast, measurements using falling-sphere viscometry are extremely difficult at such conditions because of low viscosities and small sample size.