Inclusion Criteria:  Histologically confirmed, treatment naïve EGFR-mutant NSCLC, with or without T790M resistance mutation. Presence of the sensitising EGFR-mutation (exon 19 deletion and/or exon 21 L858R) detected by an accredited laboratory. Synchronous oligo-metastatic stage IV disease (max 5 lesions) Measurable disease as defined according to RECIST v1.1 All lesions amenable for radical radiotherapy according to local judgment Age ≥18 years ECOG performance status 0-2 Life expectancy ≥12 months Adequate haematological, renal & liver function Women of childbearing potential, including women who had their last menstruation in the last 2 years, must have a negative urinary or serum pregnancy test within 7 days before enrolment. Written IC for protocol treatment  Exclusion Criteria:  Prior chemotherapy, immunotherapy, radiotherapy or therapeutical surgery for NSCLC (an exception is the resection and postoperative radiotherapy of the resection cavity of CNS or adrenal metastases) More than 5 distant oligo-metastases (any second intra-thoracic lesion will count as a distant metastasis; regional nodal metastases will not count towards 5 oligo-metastases) and more than 2 intra-thoracic lesions. Brain metastases not amenable for radiosurgery or neurosurgery Presence of leptomeningeal metastases Symptomatic spinal cord compression Extracranial metastatic locations not amenable for radical radiotherapy Currently receiving medications or herbal supplements known to be potent CYP3A4 inducers Any evidence of severe or uncontrolled systemic diseases 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 Any of the following cardiac criteria: QTcF >470 msec; Any clinically important abnormalities in rhythm, conduction or morphology of resting ECG; Any factors that increase the risk of QTc prolongation or risk of arrhythmic events Past medical history of Interstitial Lung Disease (ILD), drug-induced ILD, radiation pneumonitis which required steroid treatment, or any evidence of clinically active ILD Idiopathic pulmonary fibrosis which is a contraindication to lung radiation. History of hypersensitivity to active or inactive excipients of osimertinib or drugs with a similar chemical structure or class to osimertinib. 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 Women who are pregnant or in the period of lactation. Sexually active men and women of childbearing potential who are not willing to use an effective contraceptive method during the trial and up to 6 weeks for women and up to 4 months for men, after the last osimertinib dose.