Legislatures are critical institutions in making a democratic system function going by the assumptions that democratic governance is now a preferred system of government in many parts of the world. The most important function of legislators is policy representation which requires the advancement of the interests of their constituents in policy process. In addition, legislators have the responsibility of representing the society through their oversight function. Through the oversight function, legislators see to it that the actions and spending of the executive and other agencies of government are in line with the constitutional allowance. However, there have observations that many policies that appear good on paper have failed to achieve the expected targets after implementation. This calls to question the execution of oversight functions by the legislators. The study examined the dynamics of the Legislators performance of oversight functions in policy implementation process by the executive in Nigeria. The study was qualitative and employed in-depth interview as the method of data collection. Fifteen legislators (eight senators and seven House of Representative members) were interviewed. Narrative interpretative approach was adopted in the analysis of data. The study concluded that interference by leadership of political parties and personal interests of the legislators hindered the performance of oversight function which resulted in failure to effectively implement public policies in Nigeria.