The trends show an increase in IT Outsourcing (ITO) use. Studies also show that the relationship between the provider and service buyer is vital to the success of an ITO. Therefore, there needs to be more knowledge about which Influencing Factors (IFs) should be considered in ITO relationships. This study aims to support ITO organizations by identifying the IFs in ITO relationships and facilitating successful outcomes and relationships in buyer-provider partnerships in ITO. The authors used a systematic literature review of peer-reviewed research literature from 2010 to March 2023 on ITO and then applied grounded theory to better understand the IFs in ITO relationships. The study demonstrates that the main factors (Ifs) in an ITO relationship are Contract, Commitment, Communication, Collaboration and Trust. Additionally, the ITO services' loss of competence and control has also been observed. The knowledge provided in this research closes the research gap. For the practitioners, it delivers influencing factors that should be considered for building or developing a better relationship with their providers. The study suggests practitioners concentrate their ITO improvement initiatives on trust, contract, and communication as the three most important impacting IF factors on ITO relationship.