Abstract Petroleum Development Oman LLC (PDO) is Oman's premier oil and gas producer that operates several fields at "A". As "A" Thermal fields contain heavy oil, PDO plans to significantly increase oil recovery by increasing the steam injection which is having high energy intensity. The main objective of this paper is to demonstrate how Lean Six Sigma (LSS) approach has been utilized in improving the Thermal efficiency of the Once Through Steam Generators (OTSG) package and reduce the emissions and improve on cost efficiency. The OTSGs are designed to generate saturated steam of a nominal 80% quality, the heat input for which is produced by combusting natural gas fuel. It is contributed of around 40% of total thermal development and key enabler for thick oil production. Currently, these generators have an average thermal efficiency of around 75%, where design efficiency is between 82% to 86%. Several lean and six sigma tools and techniques were used to define the thermal efficiency improvement opportunities including; A SIPOC (Suppliers, Inputs, Process, Outputs, Customers), cost of poor quality (COPQ), RASCI, stakeholder analysis, communication Plan, detailed process mapping, Process Capability analysis, graphical and statistical analysis, Effort and Impact matrix, failure modes and effects analysis (FMEA), control plan, and leader standard work (LSW) to sustain Improvements. The utilized set of Lean Six Sigma techniques and statistical tools led to reducing the fuel gas consumed in These OTSGs by 17% and eliminated wastes and preventive maintenance cost with an expected savings of around 5% per year. A structured action closure plan has been developed and completed for this project resulted in implementing the counter measures and prepare the required fixing reports and acquire necessary approvals. Below are the results of the selected OTSG pilot. Implementation of quick win counter measures by running the OTSGs on stabilize mode, improve the gas fuel burning efficiency by 3%. While piloting the implementations in the selected OTSG, another pilot has been done for another OTSGs (two) with an additional of 2% improvement in fuel gas Thermal efficiency from 83% to 85%.