Background Plasma-activated water (PAW) is a promising innovation in agriculture, with the potential to promote seed germination and to increase plant growth and development. PAW contains reactive oxygen and nitrogen species being considered as substitute for standard fertilizers. It may enhance also plants’ natural defense mechanisms through activating several physiological and metabolic pathways. Results Oregano was foliar‑sprayed with either distilled water (control) or PAW (prepared by exposing to low temperature glow plasma for 30 min), with and without the addition of commercial fertilizer to obtain concentrations of 2.5, 5.0 and 7.5 mg/mL. PAW, both alone and combined with fertilizer, induced significant alterations in oregano metabolism, modifying bioactive compounds levels, nutrient contents, and enzyme activities, in comparison to controls (distilled water, with or without fertilizer). PAW without fertilizer induced mild oxidative stress, as evidenced by increased levels of H₂O₂ and carotenoids, although catalase activity and glutathione and sugar contents were reduced. It increased chlorophyll content, though it did not significantly increase shoot biomass. The most pronounced effects occurred with PAW combined with 7.5 mg/mL fertilizer, where heightened levels of H₂O₂, malondialdehyde, ascorbic acid, carotenoids, phenolics, sugars, and SOD activity were observed. Protocatechuic acid concentration dropped significantly in this treatment. Foliar application of PAW caused lower content of potassium, magnesium, sodium, zinc, and iron in the plants’ leaves, but it increased copper and manganese. Conclusion This is a study in which for the first-time plasma-activated water has been used as a solvent of foliar fertilizer which has been applied on oregano plants. Our findings indicate significant impact of PAW on plant biochemistry and stress responses, providing valuable data that can be used for further research purposes and future agricultural applications.