Oliveria decumbens is an aromatic plant traditionally used for treatment of infections and gastrointestinal diseases. In the present study, the volatile oil of the plant was obtained by hydrodistillation and analyzed by GC-MS. In addition, antibacterial and anti-Helicobacter pylori activities of this essential oil were determined using disc diffusion and agar dilution methods, respectively. Insecticidal activity was assessed through topical and fumigation application of the essential oil to cabbage looper larvae. Acetylcholinesterase (AChE) inhibition by the essential oil was examined using Ellman’s method. Furthermore, its cytotoxic potential against three different cancer cell lines was assessed using the MTT assay. The phenolic monoterpenoids, thymol (38.79%), and carvacrol (36.30%) were identified as major constituents of the essential oil. We observed significant antibacterial activity of the essential oil against H. pylori (MIC=20.4 µg /mL) as well as other tested bacteria, except for Pseudomonas aeruginosa. O. decumbens essential oil showed significant toxicity to cabbage looper larvae with LD₅₀ value of 52.1 µg /larva following topical and fumigant administration. O. decumbens essential oil was considerably inhibitory to acetylcholinesterase activity (IC50 = 0.117 µg/mL). Cytotoxic assay of the volatile oil resulted in IC50 = 0.065, 0.104, and 0.141 μg/mL for MCF-7, T47D and MDA- MB-231 cell lines, respectively. According to our data, this species with high concentrations of thymol and carvacrol could be considered as a natural source for pharmaceutical products.