The crucial factor in ensuring the reliability of any power transformer lies in thoroughly examining the analysis of dissolved gases (DG). However, there have been instances where the DG of the insulating oil alone has not been sufficient to assess its suitability. This study focuses on a posteriority analysis of the DG for both Mineral Oil (MO) and Natural Ester (NE). The DG analysis (DGA) results of MO and Natural Esters are compared based on the standards outlined in ASTM D3612-02. Key gases, grouped as group 1 (H 2 , CO, and CO 2 ) and group 2 (CH 4 , C 2 H 4 , C 2 H 6 , and C 2 H 2 ), are examined in terms of their retention time to determine the oil's suitability under actual transformer conditions. By segregating and interpreting the data of these Key Gases (KG) groups, a comprehensive understanding of their impact is gained, contributing to a deeper comprehension of the subject - UNDERSTANDING KEY GASES. Throughout the analysis, it is observed that the sum of key gases in both MO and natural esters remains below 1000 ppm, which is the safe limit for combustion. Consequently, the significance of this analysis underscores the importance of monitoring key gas proportions in insulating fluids to ensure transformer safety and performance.