<p>Transport phenomena involving oxytactic microorganisms driven by thermobioconvection are critically important in numerous scientific and engineering fields, such as petroleum reservoirs, enhanced oil recovery methods, contaminant migration in groundwater aquifers, biological waste management, and geothermal energy utilisation. This study examines the transport characteristics of a Casson–Williamson nanofluid moving across an exponentially expanding surface, taking into account the combined effects of buoyancy forces, Brownian motion, thermophoresis, magnetic fields, and bioconvection. The research examines the response of oxytactic bioconvective nanofluids to a periodic magnetic field. A system of coupled and highly&#xa0;nonlinear partial differential equations regulates the physical problem. The equations are subsequently converted to dimensionless form with suitable non-similar transformations. The resultant equations are numerically resolved employing the quasilinearization technique alongside the implicit finite difference approach. The numerical results are shown in graphs illustrating the influence of significant physical parameters on fluid velocity, heat transfer, mass transfer, and bioconvection. Furthermore, increasing the Casson and Williamson parameter values significantly elevates surface friction, thereby decelerating the fluid within the boundary layer. The periodic character of the magnetic field alters the momentum distribution, causing the fluid to travel toward the wall rather than within a continuous magnetic field. This reduces wall shear stress and the associated skin-friction drag. As <i>Le</i> goes from 10 to 20, the pace at which nanoparticles move from one place to another moves up by about 37%. This shows how sensitive mass transfer is to how fast nanoparticles diffuse. The reported findings demonstrate significant agreement with prior studies, hence confirming the validity and reliability of the utilised numerical methodology.</p>