Additional Study Information: The purpose of this study to learn more about whether the study drug osimertinib (TAGRISSOTM) is more effective than placebo in participants with epidermal growth factor receptor (EGFR) mutated stage IA2 or IA3 Non-Small Cell Lung Cancer (NSCLC) after complete surgical removal of the tumor. Stages IA2 and IA3 refer to tumor size. A placebo is an inactive substance or other intervention that looks the same as, and is given the same way as, an active drug or treatment being tested. The effects of the active drug or other intervention are compared to the effects of the placebo. We are also trying to better understand lung cancer and associated health problems. Osimertinib is an experimental drug. "Experimental" in this case means that the study drug has been approved by the United States Food and Drug Administration (FDA), but is not being used according to its label. Some lung cancers are due to particular mutations (changes) in the DNA (genetic material), so knowing about the mutations can help doctors decide the best treatment for their patients. One type of mutation that can occur is called the EGFR mutation. When a tumor is found to have this type of mutation, there is a class of drugs known as EGFR tyrosine kinase inhibitors (TKIs) which target the mutation to treat the patient. People with lung cancer that has the EGFR mutation may potentially be eligible for this study. Osimertinib is an EGFR TKI that targets EGFR mutations. The drug has already been approved in many countries for treatment of patients with advanced lung cancer containing EGFR mutations. Furthermore, in a similar study to this one, which uses the drug after surgery for patients with large tumors, osimertinib was shown to give a very strong benefit to patients by preventing recurrence of lung cancer.