Electric power for entire Onshore Oil and Gas operations is being supplied from Government Electric Utility through Main Intake Substation located in Seria, Brunei Darussalam. This Main Intake Substation receives bulk electric power at 66kV via two transmission lines and distributes it to various Oil and Gas facilities. The present Main Intake Substation was installed nearly 30 years ago and many of the substation equipment is nearing its useful lifetime. As a result, reliability of some of the equipment is reduced which has affected overall power availability of Main Intake Substation. Power demand for onshore operations is set to increase by more than 50% from present level in the next 10 years due to commitment to meet production targets. There is also future plan for the Government Electric Utility to upgrade their transmission system to 132kV from the present 66kV. This paper discusses the studies conducted on various options considered for upgrade of Power System to meet all these challenges. The existing system is analysed first to identify key issues affecting power availability. Following this, the key challenges are broken down into different scopes. Then various options under each scope is analysed and the best ones are selected.