The as-synthesized copolymer based on the prepared monomers and its nanohybrid were used for improving the cold flow of diesel fuel that has a vital role in meeting energy needs. The copolymer (AE) was created using the prepared monomers, by free radical solution polymerization of the prepared hexadecylmaleamide and octyloleate ester, and the polymer nanohybrid (NH) was created by emulsion polymerization of the same monomers with 1% nano-SiO2. The chemical structures of the copolymer and its nanohybrid were proved by Fourier transform infrared spectroscopy (FTIR), 1H NMR, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Through exploring the effect of the nanohybrid, before and after adding the dosage of the additives to the diesel fuel, the pour point temperature (PPT), rheological characteristics, and viscosity index were measured. The data were the best for the nanohybrid; the PPT decreased from −3 to −36 °C upon adding 10,000 ppm nanohybrid but decreased from −3 to −30 °C for 10,000 ppm copolymer. In addition, the efficiency of the additives was proved by viscosity–shear rate and shear rate–shear stress curves to give the apparent viscosity, which decreased from 124 cP for the blank to 15.74 and 12.8 cP for AE and NH, respectively; also, the yield stress decreased from 576 D/Cm2 for the blank to 541.44 and 477.9 D/Cm2 for AE and NH, respectively, at room temperature. The viscosity index increased from 116 for the blank to 119 and 121 for the copolymer and the nanohybrid, respectively. Polarizing optical microscopy was performed to show more tiny and separated wax upon adding the additives. The findings showed that delayed crystal precipitation and altered crystal shape with the NH and AE greatly reduced low-temperature viscosity and enhanced the cold flow characteristics of the diesel fuel.