This study investigates the impact of unwritten, written, and managed online feedback on students' argumentative essay writing skills, feedback quality, and domain-specific knowledge acquisition. This study uses an experimental study with experimental and control groups to see the effect of online peer feedback on students' argumentative essay writing skills. The research participants were 270 students from Universitas Pendidikan Indonesia who were randomly divided into 135 pairs and randomly assigned to 3 conditions (unwritten, written and managed peer feedback). The online peer feedback platform is the college's own Integrated Online Learning System. The instruction guide is designed within this platform. Through this research, students are asked to write argumentative essays (individual), interact in peer argumentative feedback, and finally improve based on feedback from colleagues. The results showed that the intervention in the form of an essay writing learning process guide design through online peer feedback had a positive impact on students' ability to write argumentative essays, the quality of the feedback, and the level of students' domain-specific knowledge. In general, from the three conditions, students who were in the written condition were superior to all aspects of the ability being tested. This happens because written feedback makes students more organized in expressing their input, has time to review the feedback so that it is of high quality, and is better able to understand deeply in exploring topics, so that the level of student domain knowledge increases. The implication of this research is that it provides an alternative that online peer feedback learning can be used for students' argumentation skills. The ability of scientific argumentation is needed by students in producing scientific writings in universities.