Oil & Gas flowlines are paramount for safe and reliable production of hydrocarbons, ensuring their integrity is one of the key tasks for all operators. Petroleum Development of Oman (PDO) manages around 11,000 Km of flowlines and connects to production stations more than 400 wells across more than 50 fields every year. Applications for these flowlines vary from the traditional Oil & Gas transmission to other applications such as water injection, polymer flood and steam injection. All these diverse applications involve multiple operation environments ranging from benign fields with low CO2 < 0.5 mole% and minimum H2S to very aggressive environments with CO2 and H2S concentrations well above 10 mole%, combined with a wide range of salinities and water cuts, with chlorides concentrations ranging from 5,000 to 200,000 ppm and water cuts from almost nil to more than 95%. The challenges encountered in safe and economic development of these assets across such a wide range of conditions are numerous; therefore, the material selection and designed methods to manage flowlines integrity are complex with no single solution approach to address the myriad of different conditions. This paper present two cases of metallic and non-metallic materials installed in PDO flowlines showing their historical field performance and discusses the strategy adopted for the material selection of flowlines.