Ethiopia has invested significant resources in integrated community case management (iCCM) of childhood illness. In Oromia Region, iCCM scale-up was phased in, allowing for comparisons between districts providing iCCM and routine services. We assessed the determinants of utilization of health extension workers (HEWs) delivering iCCM services at rural health posts by caregivers of sick, under-five children in a cross-sectional survey. We found low utilization of HEWs with only 9.3% of caregivers of a child sick with diarrhea, fever, and/or pneumonia in the previous 2 weeks taking their child to HEWs in both iCCM and routine areas. There was a higher likelihood of utilization of HEWs in iCCM areas (OR: 1.44; 95% CI: 0.97–2.12; P = 0.07), but this effect disappeared after accounting for confounders. In iCCM areas, maternal education, illness type, and distance were associated with utilization. Perceptions of illness severity and service quality were the primary reasons given for not utilizing the health post. Our findings suggest that though iCCM is reaching some vulnerable populations, there remain significant barriers to use of HEWs delivering iCCM services. Efforts for demand generation and minimization of remaining barriers are urgently needed for the sustained success of the iCCM strategy in Ethiopia.