IntroductionCancer remains a leading cause of death globally, with patients frequently experiencing malnutrition due to both the disease and its treatment, which negatively affects their quality of life and treatment outcomes. Oral nutritional supplements (ONS) provide a noninvasive solution to improve nutritional status, but poor patient adherence limits their effectiveness. Studies on ONS adherence vary in their definitions and assessment tools, creating inconsistencies. A scoping review is essential to synthesize these studies and establish a foundation for future research and clinical practice.MethodWe systematically searched six databases, including Web of Science, PubMed, and Scopus, up to August 2024. Our criteria focused on oncology patients, ONS interventions, and outcomes related to adherence definitions, assessment methods, and adherence rates.Results37 studies from 2005 to 2024 met the inclusion criteria. Definitions of ONS adherence and assessment methods vary widely, with the most common definition being the ratio of actual intake to the recommended amount. The assessment tools included self-reported ONS diaries, and MMAS scores, among others. Adherence rates also vary significantly, with some studies reporting a decline in adherence over time.ConclusionThe lack of standardized definitions and assessment methods for ONS adherence across studies hinders comparability. Future research should focus on developing standardized, comprehensive adherence assessment tools that incorporate both quantitative and qualitative data. This would allow for a better understanding of adherence factors and enable more targeted interventions to improve long-term adherence in cancer patients.