Information technology outsourcing (ITO) studies have been the focus of researchers for decades. One topic has been the factors and impact of ITO on organizations. Inconclusive findings about relationships between the levels of ITO and ITO success demands further exploration on this issue. In Thailand, the National Electronics and Computer Technology Center (NECTEC) reported that the financial and banking industry has the highest spending on computer services primarily by outsourcing the core banking and service management system. Nonetheless, research on ITO in Thailand’s financial industry has been limited. This study is among the first to empirically investigate the relationship between the levels of ITO, defined as the percentage of IT activities and components selected to be outsourced, and ITO success, defined by the benefits received from outsourcing in Thailand. A three-pronged approach is used in this study. It began with a comprehensive literature review. This was followed by in-depth expert interviews with representatives from financial institutions, academia, and outsource vendors, all of who offered opinions that shaped the development of the research model. A survey was then carried out with 428 IT personnel from the Thai financial sector. The results were analyzed using structural equation modeling (SEM). Contrary to previous researches, this study found that internal uncertainty and internal IT skills were two factors that do not affect the levels of ITO, while transaction costs and specific assets do. In addition, it was found that levels of ITO have impacts on ITO success. In a real world application, this study focuses on the different levels of ITO implementation among Thailand’s top five banks and their effects on ITO success. The findings increase researchers’ understanding regarding the factors that affect the levels of ITO and suggest that practitioners must consider the levels of ITO, in addition to other factors, when making ITO decisions. The framework can help practitioners optimize ITO practice for successful ITO outcomes.