This paper provides a practical guide to due diligence in upstream oil and gas transactions, covering the full overview from informal pre-signing review through formal post-signing investigation. It examines how transaction structure shapes diligence scope and risk allocation across the three core workstreams of corporate, title, and environmental diligence, and explains how title standards, defect notices, and cure mechanisms operate under modern purchase agreements. Written for practitioners who need both doctrinal grounding and real-world tools, the paper also offers strategic guidance on seller-side readiness, execution under time pressure, and privilege and confidentiality considerations, and includes a sample due diligence checklist and title defect notice as practical takeaways.