Oregano (Origanum vulgare L.) is a traditional medicinal plant that is widely used for its health benefits due to its pharmacological properties, such as antimicrobial, carminative and antioxidant activities. Heat stress caused by climate change severely restricts the growth and yield of medicinal plants. Limited studies show that the application of natural biostimulants is an innovative and cost-effective alternative to synthetic fertilizers for enhancing plant growth and productivity. Also, little is known about the effect of nano-fertilizers on the growth and biochemical characteristics of medicinal plants. Thus, the current study investigated the efficacy of cancer bush (Lessertia frutescens L.) leaf extract (CLE) (2%) enriched with biosynthesized zinc oxide nanoparticles (ZnO-NPs) to improve the growth and biochemical attributes of oregano subjected to heat stress (38 ˚C for 2 hours for 6 days). The treatments used were CLE, CLE + ZnO-NPs 25, 50 and 75 mg/L folia spray and plants that were folia-sprayed with water were used as control. The randomized complete block design (RCBD) was used in this study and data were collected at 7-day intervals during the experiment. The results showed that heat stress reduced the growth and yield and the physiological and phytochemical attributes of oregano. However, the application of CLE + ZnO-NPs enhanced the growth and biochemical composition of heat-stressed plants. Both 50 and 75 mg/L CLE + ZnO-NPs treatments increased the growth and yield attributes, essential oil yield, total chlorophylls and carotenoids, ascorbic acid content, total phenolic content, antioxidant enzyme activities (catalase, ascorbate peroxidase, and superoxide dismutase activities) and reduced malondialdehyde and electrolyte leakage in oregano plants compared to other treatments. Therefore, CLE-enriched ZnO-NPs, especially at 50 and 75 mg/L concentrations, can be recommended for improving the productivity and biochemical characteristics of oregano under heat stress.