This article highlights the influence of thermal radiation, inclined magnetic field, and stratification on the third-grade nanofluid with swimming gyrotactic microorganisms. The fluid is flowing past a horizontal cylinder exposed to an inclined magnetic field. The boundary layer system of equations is reconstructed into ordinary differential equations using similarity transformations. The system of equations is resolved using the Optimal Homotopy Analysis Method (OHAM) and the results are visualized numerically and graphically. Physical properties have been taken into consideration and the repercussions of different parameters on these properties are analyzed through contour plots. The following upshots are prominent: (i) radiation augments diffusive energy to the system enhancing the temperature along with the heat transfer rates, (ii) the more inclined magnetic field causes more decrease in velocity (iii) the bioconvective aspect introduced due to the addition of microorganisms altered the transfer rates of microorganisms positively, and (iv) nanoparticles evidently brought down the concentration through Brownian motion effects and the overall influence of these particles hindered mass transport rates.