The prevalence of malnutrition is high in gastrointestinal (GI) cancer patients. The use of oral nutrition supplementation (ONS) as part of patients' nutritional therapy seems to be effective in the improvement of nutritional status. Nevertheless, oncology patients, experience several symptoms that negatively affect their compliance with ONS products. Τhe aim of this systematic review is to examine the factors affecting compliance with ONS in patients who underwent GI cancer surgery and/or adjuvant treatments. A systematic search was conducted to identify studies published until June 2023 that assessed compliance to ONS in GI cancer patients. Eleven studies fulfilled the eligibility criteria and were included in the analysis. Postoperative compliance with ONS among GI cancer surgery patients ranged between 26.2% and 71.1%, whereas in GI cancer patients receiving chemotherapy the average reported rate was 90.2%. The main reasons for noncompliance were the presence of GI symptoms, such as early satiety, bloating, and diarrhea after ONS consumption, as well as taste alterations that result in aversion to the provided ONS. Frequent monitoring of these patients is crucial in order to record adverse effects, identify patients that are in need of personalized guidance at an early stage and motivate them to follow their ONS plan.