Employing x-ray computed tomography (μ-CT), we examined the impact of an oil mallee (OM) biochar, at concentrations of 0%, 1% and 5% (w/w), on soil structural traits in three soil types (Vertisol, Ferralsol and Arenosol). The biochar was pyrolysed at a maximum temperature of 550°C, sieved to between 250 μm and 2 mm prior to amending the soils, had an internal porosity of 75% (v/v) and an organic carbon (Corg) content of 60%. Soil structure was quantified, at a resolution of 70 μm, by measuring μ-CT porosity, mean pore radius, fractal dimension and connectivity of the pore space. Addition of 5% OM biochar resulted in higher μ-CT porosity (i.e. ≥70 μm) in the Vertisol (p < 0.001), averaging 7.5, 9.1 and 13.4%, respectively, for 0, 1 and 5% biochar, with the Ferralsol having corresponding μ-CT porosities of 6.2, 6.5 and 10.9%, the difference also being significant (p = 0.03) for 5% OM biochar amendment. Significant increases (p < 0.05) in connectivity of the largest pore and mean pore radius were observed in all three soils containing 5% OM biochar and also in the Arenosol with 1% OM biochar. Over a 15-month incubation the Ferralsol containing 5% OM biochar showed increased μ-CT porosity (p < 0.05) and fractal dimension (p < 0.05), the latter indicating greater homogeneity of pore space distribution. Addition of OM biochar significantly decreased the rate of water loss through evaporation and drainage of all incubated soils (p < 0.05) and reduced volumetric water content at field capacity in the two clay-rich soils. Soil with 1% OM biochar showed an increase in the rate of drainage in the Vertisol (p < 0.05 at −0.02 MPa) and a 20% increase in the available water content of the Arenosol (p < 0.05). It is hypothesised that the highly porous structure of biochar may have contributed to the changed water retention characteristics of the soils. This study highlights the potential for OM biochar to modify the structural characteristics of contrasting soils.