Sandy soils with high alkalinity are characterized by low copper (Cu) contents that lead to many deficiency symptoms in plants. Cu deficiency in plants can be corrected using several cheap Cu sources. Nevertheless, the effects that novel sources, such as Cu nanoparticles (NPs), have on plants remain poorly studied. In the present work, we investigated the effect and efficiency of Cu supplementation to onion (Allium cepa L.) plants using Cu sulfate, chelate, or NPs, and compared their effects on bulb quality, yield, and contents of phytochemicals. Two successive seasons (2018/2019 and 2019/2020) of field experiments were conducted in newly reclaimed sandy soils, where plants were sprayed with either 10 ppm CuO NPs, 20 ppm CuSO4·5H2O, or 20 ppm of Cu chelates. Overall, Cu deficiency (control) resulted in a significant decrease in yield and all quality traits of onion plants. CuO NPs treatment significantly enhanced growth parameters, including plant height, number of leaves, fresh and dry weight, yield, and bulb quality, compared with Cu sulfate and chelates. This was also the case regarding chemical constituents such as macro- and micro-nutrients, total soluble solids, phytochemical compounds, vitamins, and amino acids. Although Cu sulfate is the cheapest form used for Cu supplementation, results of the present study suggest that CuO NPs was not only safe to use, but also was the treatment that led to the highest onion yield and quality.