COordinated Oral health Promotion (CO-OP) Chicago is a two-arm cluster-randomized trial with a wait-list control. The primary aim is to evaluate the efficacy of an oral health community health worker (CHW) intervention to improve oral health behaviors in low-income, urban children under the age of three years. Exploratory aims will determine cost-effectiveness, and if any CHW intervention impact on child tooth brushing behaviors varies when CHWs are based out of a medical clinic compared to a community setting. This paper describes progress toward achieving these aims. Participating families were recruited from community social service centers and pediatric primary care medical clinics in Cook County, Illinois. Sites were cluster-randomized to CHW intervention or usual services (a wait-list control). The intervention is oral health support from CHWs delivered in four visits to individual families over one year. The trial sample consists of 420 child/caregiver dyads enrolled at the 20 participating sites over 11 months. Participant demographics varied across the sites, but primary outcomes values at baseline did not. Data on brushing frequency, plaque, and other oral health behaviors are collected at three timepoints: baseline, 6-, and 12-months. The primary analysis will assess differences in caregiver-reported child brushing frequency and observed plaque score between the two arms at 12-months. The trial is currently in the active intervention phase. The trial's cluster-randomized controlled design takes a real-world approach by integrating into existing health and social service agencies and collecting data in participant homes. Results will address an important child health disparity. ClinicalTrials.gov identifier: NCT03397589. CLINICAL TRIAL REGISTRATION: University of Illinois at Chicago Protocol Record 2017-1090. National Institutes of Dental & Craniofacial Research of the National Institutes of Health (NIDCR) Protocol Number: 17-074-E. NCT03397589.