Inclusion Criteria:

          1. Provision of informed consent prior to any study specific procedures

          2. Male or female must be > 19 years of age.

          3. Locally advanced or metastatic NSCLC, not amenable to curative surgery or radiotherapy
             with local confirmation of the presence of the EGFR exon 20 insertion mutation

          4. Disease progression while on standard chemotherapy (platinum doublet chemotherapy or
             single-agent chemotherapy in selected patients)

          5. Eastern Cooperative Oncology Group (ECOG) performance status 0-1.

          6. Patients must have a life expectancy ≥ 12 weeks

          7. Females should be using adequate contraceptive measures, should not be breast feeding
             and must have a negative pregnancy test prior to start of dosing if of child-bearing
             potential or must have evidence of non-child-bearing potential by fulfilling one of
             the following criteria at screening:

          8. Male patients should be willing to use barrier contraception

          9. Patient is willing and able to comply with the protocol for the duration of the study
             including undergoing treatment and scheduled visits and examinations including follow
             up.

         10. At least one measurable lesion

         11. Provision of archival FFPE tissue

         12. Provision of informed consent for translational genetic research

        Exclusion Criteria:

          1. Involvement in the planning and/or conduct of the study (applies to both sponsor staff
             and/or staff at the study site)

          2. Previous treatment with osimertinib (3rd generation EGFR TKIs such as olumtinib,
             EGF816 etc)

          3. Treatment with an investigational drug within five half-lives of the compound

          4. Patients currently receiving (or unable to stop use prior to receiving the first dose
             of study treatment) medications or herbal supplements known to be potent inhibitors of
             CYP3A4 (at least 1 week prior) and potent inducers of CYP3A4 (at least 3 week prior)
             (Appendix A). All patients must try to avoid concomitant use of any medications,
             herbal supplements and/or ingestion of foods with known inducer/inhibitory effects on
             CYP3A4.

          5. Any unresolved toxicities from prior therapy greater than Common Terminology Criteria
             for Adverse Events (CTCAE) grade 1 at the time of starting study treatment with the
             exception of alopecia and grade 2, prior platinum-therapy related neuropathy.

          6. Any evidence of severe or uncontrolled systemic diseases, including uncontrolled
             hypertension and active bleeding diatheses, which in the investigator's opinion makes
             it undesirable for the patient to participate in the trial or which would jeopardise
             compliance with the protocol, or active infection including hepatitis B, hepatitis C
             and human immunodeficiency virus (HIV). Screening for chronic conditions is not
             required.

          7. Patients with symptomatic CNS metastases who are neurologically unstable; however,
             those with asymptomatic CNS metastases who do not require steroids for at least 4
             weeks prior to start of osimertinib are eligible.

          8. Past medical history of ILD, drug-induced ILD, radiation pneumonitis requiring steroid
             treatment, or any evidence of clinically active ILD

          9. Inadequate bone marrow reserve or organ function

         10. QTc prolongation (mean resting corrected QTc > 470 msec)

         11. Refractory nausea and vomiting, chronic gastrointestinal diseases, inability to
             swallow the formulated product or previous significant bowel resection that would
             preclude adequate absorption of osimertinib

         12. History of hypersensitivity to osimertinib (or drugs with a similar chemical structure
             or class to osimertinib) or any excipients of these agents

         13. Males and females of reproductive potential who are not using an effective method of
             birth control and females who are pregnant or breastfeeding or have a positive (urine
             or serum) pregnancy test prior to study entry

         14. Judgment by the Investigator that the patient should not participate in the study if
             the patient is unlikely to comply with study procedures, restrictions and requirements

         15. Previous allogeneic bone marrow transplant.

         16. Non-leukocyte depleted whole blood transfusion within 120 days of the date of the
             genetic sample collection.