Loose sandy soils in arid regions such as Varzaneh face challenges of high erodibility and low mechanical strength. Microbially induced calcium carbonate precipitation (MICP) is a sustainable bio-based solution for soil improvement. However, deep application of this technology faces a major obstacle: the aerobic bacterium Sporosarcina pasteurii shows a sharp decline in metabolic activity and carbonate precipitation at depths beyond a few centimeters due to insufficient oxygen availability. In the present study, the controlling role of oxygen was investigated through active gas supply (compressed air containing 21% oxygen and pure oxygen with 99.5% purity). Experiments were conducted on columns of Varzaneh sand (SP, D₅₀ = 0.155 mm) under three treatment regimes: treatment without gas, treatment with compressed air, and treatment with pure oxygen. The results showed that injection of compressed air increased the unconfined compressive strength (UCS) by up to 50%, and injection of pure oxygen increased it by more than 100% compared to the control group. The average precipitated calcium carbonate content at a depth of 30–45 cm was less than 1% by weight for the no gas treatment, about 4% for the compressed air treatment, and about 7% for the pure oxygen treatment. Scanning electron microscopy (SEM) images revealed that, in the presence of oxygen, spherical and agglomerated calcite crystals formed with effective interparticle bonding, whereas in the absence of oxygen, scattered rhombohedral crystals were observed. The findings confirm that oxygen plays a decisive role in deep bio improvement of soil, and the unsaturated conditions of arid regions offer a strategic advantage for field application of this technology.