The main purpose of this study is to assess if olomorasib in combination with pembrolizumab is more effective than the pembrolizumab and placebo combination in part A in participants with resected KRAS G12C-mutant NSCLC and to assess if olomorasib in combination with durvalumab is more effective than the durvalumab and placebo combination in part B in participants with unresectable KRAS G12C-mutant non-small cell lung cancer. The study may last up to 3 years for each participant.