This article presents a comprehensive, publication-ready examination of migrating enterprise database systems from Oracle to PostgreSQL. It synthesizes theoretical foundations, practical migration techniques, cost analyses, reliability considerations, and operational frameworks derived from contemporary documentation, migration tooling, cloud provider materials, and technical whitepapers. The study begins by establishing the technical equivalence and divergences between Oracle and PostgreSQL in areas critical to enterprise systems: transactional guarantees, concurrency control, data types, stored procedure ecosystems, and replication and high-availability mechanisms (Database Reliability Review, 2023; PostgreSQL, 2023). It then develops a structured methodology for planning and executing migration projects, combining schema conversion, code translation, performance tuning, data validation, and cutover strategies informed by migration tools such as Ora2Pg and established cloud migration practices (Ora2Pg, 2023; Database Technology Trends, 2022). The article evaluates cost and operational impacts, contrasting proprietary licensing and maintenance costs with open-source licensing and cloud-managed service models, with particular attention to AWS RDS and comparative cloud offerings (Cloud Database Solutions: A Comparison of PostgreSQL and Oracle RDS, 2023; AWS, 2023). Using both theoretical models and synthesized empirical observations from existing migration reports and whitepapers, the results section describes expected cost savings, performance risks, and typical time-to-stabilization trajectories following migration (Natti, 2023; Oloruntoba & Omolayo, 2024). The discussion provides deep interpretation of trade-offs—technical, economic, and organizational—while outlining limitations of current tools and future research directions including formal verification of translated PL/SQL, automated performance regression detection, and tighter operational integration with cloud-native service meshes (Nuijten & Barel, 2023; Ravikumar et al., 2017). The conclusion offers a practical decision framework and a prescriptive checklist for enterprises considering an Oracle-to-PostgreSQL migration, emphasizing rigorous testing, staged rollouts, and continuous performance governance. Keywords: database migration, PostgreSQL, Oracle, cost reduction, Ora2Pg, cloud RDS, ACID compliance.