To generate oregano essential oil, the leaves and flowering tops of the Origanum vulgare plant go through the process known as steam distillation. This essential oil is known for its antibacterial activity. Bacterial biofilms are microbial communities attached to inert surfaces or tissues and encapsulated in complex matrices. Planktonic bacteria reversibly attach to surfaces, form microcolonies, and generate polymeric matrices around biofilms. Bacteria in biofilms provide bacteria with a safer way to reproduce and survive. This research tests the antibacterial activity and effect on the biofilm formation of Oregano essential oil. The antibacterial activity and effect on biofilm formation were tested against five bacterial strains, including Escherichia coli ATCC 14169, Escherichia coli ATCC 25922, Staphylococcus aureus NCTC 12393, Staphylococcus aureus ATCC 25923, and Staphylococcus aureus ATCC 6538. The concentrations of oil that were used in this research were 100%(v/v), 75%(v/v), 50%(v/v), and 25%(v/v). The best antibacterial effect was achieved against Staphylococcus aureus NCTC 12393 at 25%(v/v) of oil concentration. While performing the experiment, a variety of oregano oil concentrations had significant results for further tests to be performed.