One of the indispensable assets of electrical systems and its most expensive components are power transformers. Condition assessment of power transformers is crucial and must be performed precisely to detect any abnormal conditions and prevent failures. Most transformer condition assessment relies on evaluating the insulating oil quality that achieves the cooling inside the transformer and the electrical insulation role. Hence, insulating oil performance is an important term for the reliable operation of power transformers. Under field aging, there is critical degradation in the chemical properties of the transformer oil, which may cause the occurrence of arcing, thermal, and partial discharge fault types. One of these chemical degradations caused by transformer aging is the generation of dissolved gases. Hence, dissolved gas analysis (DGA) is considered an in-service common and effective technique for detecting oil condition assessment under field aging. In addition, this technique can diagnose fault types. This paper studies the effect of oil aging on dissolved gas generation with a novel view that can diagnose not only the transformer oil condition but also the transformer paper, on-load tap changer, and observation system. For this purpose, it presents an analysis of the generation of dissolved gases for four old transformers of the same oil type (Diala B) and different ratings that exist in the Egyptian Electricity Transmission Company (EETC). This analysis of transformer oils was studied from 2010 to 2021. Based on dissolved gas concentrations such as hydrogen and oxygen, the water content is calculated and compared during the transformer study durations.