This scoping review was undertaken as the first stage of development of the Core Outcome Sets for Early Prevention of Obesity in CHildhood (COS-EPOCH). The aim of this review is to identify the outcomes collected and reported in randomized controlled trials of early childhood obesity prevention interventions. A systematic scoping review was undertaken following published guidelines. Trial registries and Medline were searched, and records retrieved were screened by two reviewers. Included trials aimed to prevent childhood obesity in the first 5 years of life and were randomized. Data were extracted using a standardized form. Outcomes were assigned to outcome domains, and similar definitions within each domain were merged, based on key literature and expert consensus. Outcome and domain frequencies were estimated and presented in outcome matrices. Eighteen outcome domains were identified from 161 included studies: "anthropometry," "dietary intake," "physical activity," "sedentary behaviour," "emotional functioning/wellbeing," "feeding," "cognitive/executive functioning," "sleep," "other," "study-related," "parenting practices," "motor skill development," "environmental," "blood and lymphatic system," "perceptions and preferences," "quality of life," and "economic," "oral health." The most frequently reported outcome domain was anthropometry (92% of studies), followed by dietary intake (77%) and physical activity (60%). 221 unique outcomes were identified, indicating a high degree of heterogeneity. Body mass index was the only outcome reported in >50% of studies. The considerable heterogeneity in outcomes supports the need for the development of COS-EPOCH.