Purpose <p>The effect of foliar-sprayed silicon nanoparticles at various concentrations (0, 50, 100, and 150 mg L^− 1) was evaluated in reducing fluoride toxicity (0, 200, and 400&#xa0;mg kg^− 1 soil) on two contrasting oat cultivars (JHO-99-1 and JHO-822).</p> Methods <p>The silicon nanoparticles were biosynthesised by using <i>Polyalthia longifolia</i> leaves, and were then characterised by different methods, UV-VIS spectrophotometer, Zeta sizer and potential, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscope. Atomic absorption spectroscopy was used to evaluate the silicon content and a fluoridometer was used to measure the fluoride concentration.</p> Results <p>The addition of silicon nanoparticles improved the growth, silicon content, non-enzymatic and enzymatic antioxidant activities, which helped mitigate oxidative stress and reduced the fluoride accumulation in oat cultivars up to (200&#xa0;mg kg^− 1 soil) concentration, however higher concentration (400&#xa0;mg kg^− 1 soil) was harmful and resulted in a significant growth reduction. Silicon content improved in both oat cultivars roots and leaves with increase in silicon nanoparticles concentration, suggesting that fluoride stress had minimal or no impact on silicon accumulation.</p> Conclusion <p>According to the current study, using silicon nanoparticle as a foliar spray to oat cultivars can effectively reduce the toxicity of fluoride by raising the silicon content and reducing the fluoride content in a dose-dependent way. The ideal concentration was 150 mg L^− 1 of silicon nanoparticles, which reduced the fluoride content in both oat cultivars.</p>