Porous ultra-high-temperature ceramics (UHTCs) have been studied for their use in transpiration cooling of the leading edges of hypersonic vehicles. The envisioned design consists of a layered hybrid material: a dense UHTC substructure with channels providing coolant fluid to a partially sintered, permeable surface layer to evenly distribute coolant across the leading-edge surface. To assess sintering during the fabrication of zirconium diboride (ZrB2) for the surface layer, two size grades of commercially available ZrB2 powders were heated in air to 450 – 540 °C to vary oxygen content in the powder from 0.5 – 5.3 wt%O. Monitoring of density during hot-pressing showed that densification during initial stage sintering was plastic flow, with grain boundary diffusion from 1730 °C and volume diffusion from 1850 °C. The onset of rapid densification during hot-pressing of oxidised powders was delayed until after the onset of volume diffusion and prevented entirely with 5.3 wt%O in the fine power and 2.2 wt%O in the coarse powder up to 2000 °C/ 24 MPa. Increasing powder size increased permeability from 0.79 m2 to 2.59 m2 in 33 vol% porous ZrB2 hot-pressed at 1700 °C/ 12 MPa due to an increase in pore size from 0.66 to 1.1 µm. Increased surface transport also increased pore size, allowing for permeability of 1.9 x 10-14 m2 at 31 vol% porosity. The resulting reduction in pore surface area prevented further densification of the partially sintered microstructures at intended application temperatures of 2000 °C. The elastic modulus, flexural strength, and thermal conductivity of partially sintered ZrB2 increased linearly with density after the initial development of properties above 40 vol% porosity. Flexural strength and thermal conductivity were improved due to the expansion of inter-particle neck area when hot-pressing with oxidised powders...