The purpose of this study is to evaluate efficacy and safety of osimertinib (tablet) in combination with Dato-DXd (i.v. infusion) compared with osimertinib (tablet) monotherapyas a first-line therapy in participants with locally advanced or metastatic EGFRm (Ex19del and/or L858R) NSCLC. Study details include: 1. The study duration will be event-driven, with an estimated duration of approximately 9 years. 2. Participants may receive study treatment until disease progression, unacceptable toxicity, or other specific discontinuation criteria are met. 3. The visit frequency will be every 3 weeks during the treatment period. Note: Participants on osimertinib treatment(osimertinib only arm or who have discontinued Dato-DXd while are still receiving osimertinib) are required to attend visits to perform assessments every 6 weeks from Cycle 7 until Cycle 17 and then visits every 12 weeks until disease progression or IP discontinuation. Participants who are receiving osimertinib + Dato-DXd are still required to attend visit to perform assessment every 3 weeks (q3w) per SoA.