The Oil and Gas sector has been the main stay of the Nigerian economy since independence in 1960. It is one of the choice sectors in Nigeria, unfortunately a review of financials of firms in that sector does not show significant growth in profitability over the years. This study was therefore conducted to ascertain the determinants of financial performance of the listed oil and gas companies in Nigeria. The ex-post facto research design technique was adopted as the study was experimental. This was to enable the researchers examine the influence of both the internal and external variables on the financial performance of the listed oil and gas companies in Nigeria. The dependent variable was: Returns on Assets (ROA) and while the independent variables were: Internal Variables-Capital Structure (CAS), Liquidity (LQ), Size (SZ), Age (AG), Sale Revenue Growth (SRG), Profit Margin (PM) and Tangibility (TAN) and the External Variables-Inflation Rate (IFR) and Growth Rate of Real Gross Domestic Product (RGDP). The Statistical Package for Social Sciences (SPSS) was used to analyze the data collected and multiple linear regression was used to test the hypotheses at 5% level of significance. Out of a total of 12 listed Oil and Gas companies as at 31st December 2018, 6 were selected using judgmental sampling technique. From the regression results, CAS, SZ, AG and PM were significant in the model. Adjusted R-Square showed that 80% variations in ROA was explained by the influence of internal and external variables (CAS, LQ, SZ, AG, SRG, PM, TAN, IFR and RGDP).