Tax evasion and avoidance are both phenomena that are probably as old as taxation itself. Literature revealed that tax avoidance and evasion represent some of the perplexing problems facing the Nigerian economy. The study adopts a quantitative research design, employing a standardized questionnaire as the primary research instrument. The study population comprises personnel from the Oyo State Internal Revenue Service. The findings revealed a significant positive relationship between tax evasion and IGR, indicating that tax evasion negatively impacts revenue generation. Similarly, tax avoidance is found to have a significant negative relationship with IGR, implying its detrimental effect on revenue. The study concluded that resistance towards tax obligations leads to the disturbance of the government's essential services, such as health services, education, sanitation, transportation, and infrastructure. The study recommends that urgent steps should be taken by public office holders to live up to expectation when it comes to transparency, accountability and the war against corruption.