This research focuses on the interaction between pairs of teachers who carry out Reciprocal Peer Observation (RPO) practices. The aim is to identify how teachers learn by building knowledge collaboratively during their participation in feedback meetings (the third stage of the RPO cycle). From an initial sample of 400 voluntary in-service teachers, audio files recorded during feedback meetings are collected. Teacher pair interaction is analyzed in 94 meetings using a system of categories and subcodes. Discourse moves related to collaborative knowledge building (CKB) and non-CKB are analyzed to check if there are differences between the two roles carried out (observer and observee). The study combines descriptive and inferential analysis, and results indicate that there is a greater use of discourse moves related to CKB. Differences between observer and observee roles are identified, indicating that RPO can be an adequate practice for teacher learning because it allows each teacher to take advantage of the opportunities of both roles. It is concluded that enriching feedback meetings with the use of informed knowledge, support of tools, and quality feedback can promote even more CKB in RPO feedback meetings and therefore continue to move forward to practices in which teachers can collaboratively learn from each other.