Partial Discharge (PD) activity at the interface of oil pressboard begins before full surface discharge. It is visualized as white marks or tree like structures on the surface of aged pressboards. The PD activity can cause irreversible damage to the insulation material due to its thermal and chemical actions. Thus, extensive study is necessary to classify the PD patterns within oil, inside the pressboard as well as at the interface. The nature of PD characteristics depend on various parameters such as type of oil, pressboard material, electrode configuration, gap spacing, etc. In this work, PD characteristics of oil-pressboard interface are studied using mineral oil (Nytro 10XN), synthetic ester (MIDEL 7131) and natural ester (FR3) with different electrode configurations and gap spacing. The experiments were conducted according to the standard IEC 60641-2 and the PD patterns were recorded with the help of Omicron PD detector (MPD-600). Care was taken while preparing the pressboard samples and their impregnation process. In order to compare the PD characteristics, Partial Discharge Inception Voltage (PDIV) and Phase Resolved Partial Discharge (PRPD) patterns were recorded. Initially, the effects of gap electrode configurations and gap spacing were studied with mineral oil- pressboard interface with the help of recorded PDIV magnitude and PRPD patterns. Secondly, the investigations were carried out for different types of transformer oils. The PRPD patterns were analyzed using the characteristics such as apparent charge and repetition rates. It was observed that pressboard material immersed in mineral oil and natural esters has lower apparent charge and repetition rates as compared to synthetic ester at the same voltage levels. © 2015 IEEE.