Transformers with early defect detection experience fewer expensive, unplanned outages. The most accurate and dependable method for assessing the condition of a transformer is dissolved gas analysis. As an instrument for diagnosis, dissolved combustible gases in oil are used to evaluate the transformer’s condition. Under the influence of different faults condition, the mineral oil in the transformers breaks down into a variety of gas compounds, including H2, O2, N2, CO, CO2, CH4, C2H6, C2H4, and C2H2. The gases evolved in the transformer can be investigated using a variety of DGA approaches, including the key gas ratio method, Dornenburg’s method, the Roger ratio method, the Duval triangle method, the IEC ratio method, etc. In this paper, DGA tests should be performed on various real-time aged transformer oil collected from different rating transformers like 10 MVA (2015–9 years), 10 MVA (2016–8 years), 630 KVA (2017–7 years) and 315 KVA (2019–5 years). In order to determine the fault conditions DGA method like Roger’s ratio, IEC ratio and key gas method are applied on oil samples collected from four transformers. Key gases method shows the Partial discharge in oil in each four samples. Roger’s ratio, shows the Partial discharge in Sample (A) and (B), and in sample (C) and (D), slight overheating < 150 ℃ fault shows. IEC ratio method shows Partial discharge in oil in sample (A) and (B), and in Sample (C) and (D) shows the Thermal fault Ta, 150 ℃ < Temp < 300 ℃.