Metal oxide nanoparticles (NPs) such as CuO, and ZnO can have significant effects on plant root cells, depending on factors like concentration, size, and surface properties of the nanoparticles. These metal oxide nanoparticles can cause toxic and genotoxic effects on plant root cells, affecting overall plant growth and development. This study evalaute the cytological and oxidative effect of varying concentration (100v/v, 80 v/v, 40 v/v and 20 v/v) of CuO-NPs, ZnO-NPs and CuO/ZnO-NPs on Onion (Allium cepa L) root tip cells. The synthesized CuO-NPs, ZnO-NPs and CuO/ZnO-NPs were of average particle sizes of 12.78nm, 13.76 nm and 13.35 nm respectively as revealed from the XRD study. The SEM analysis showed that CuO-NP and ZnO-NP appeared to have an irregular and granular particle shape, while the CuO/ZnO-NPs appeared to have a flower like structure. The chromosomal aberrations of nanopartcle treated Allium cepa L exhibited anormolies. The study revealed that at higher concentration (100% and 80%) of CuO-NPs, ZnO-NPs and CuO/ZnO-NPs exhibited higher chromosomal damage as evidence from the higher abnormality percentage (72.50 and 56.2%), (69.10% and 61.20%), and (81.10% and 74.25%) respectively. The effect of concentration revealed that the mitotic index was concentration dependent with 20v/v concentration showing reduced mitotic activities across all samples. The study reveals that the uncontrolled application of these nanoparticles may have harmful effect on plants cell growth when exposed in polluted soil.