Online learning overcomes space and time constraints; however, it presents challenges in teacher-learner interaction. This study explores the potential of feedback in online formative assessment (OFA) to enhance such interactions. This study identified different types of feedbacks and analyzed the characteristics of interactions between teachers, technology, and learners. The research participants were 22 pre-service physics teachers in Seoul, Korea. They used OFA in the demonstration of online classes, and the types of feedback were classified through their OFA utilization data, instruction plan, and observation data, with interviews supporting the results. The study classified each feedback type in detail through elaborated feedback (EF). For multiple-choice questions, the item-specific type was more effective than the answer-until-correct or summary types in providing customized feedback and adjusting classes. The feedback types of constructed-response questions focused on the specific content or subject integration but challenging to utilize for automated feedback. Based on these feedback types, the study identified the characteristics of interactions between teachers and students mediated by technology through demonstration observation. Social interaction between teachers and learners occurred via technology when the correct response was immediately provided per item. In this type, technology played a strong mediating role in the utilization of automatic feedback, and the teacher played a leading role in guiding the learner. Furthermore, EF provided per item enabled meaningful interactions. The study underscores the crucial role of teachers in providing meaningful support through OFA feedback in an evolving educational environment.