This study attempts to investigate challenges faced by EFL high school teachers and students implementing online peer-assessment (OPA) in oral presentation. Participants were 6 high school teachers and 193 students in four provinces in the Mekong Delta of Vietnam. The qualitative data and quantitative ones were collected by using a questionnaire and semi-structured interviews. The findings show that EFL high school students and teachers encountered a number of challenges in utilizing OPA. Specifically, students’ low individual accountability and substantial time and effort in assessment organization and management are perceived as the biggest challenges of teachers while students’ lack of experience, language ability, and knowledge is the major obstacle of students. Based on these findings, pedagogical implications are made for implementing OPA in oral presentation.