This study describes the socio-cognitive dynamics of collaborative online knowledge-building discourse among Dutch Master of Education students from the perspective of openness. A socio-cognitive openness framework consisting of four social and four cognitive components was used to analyze contributions to online collective knowledge building processes in two Knowledge Forum® databases. Analysis revealed that the contributions express a moderate level of openness, with higher social than cognitive openness. Three cognitive indicators of openness were positively associated with follow-up, while the social indicators of openness appeared to have no bearings on follow-up. Findings also suggested that teachers' contributions were more social in nature and had less follow-up compared to students' contributions. From the perspective of openness, the discourse acts of building knowledge and expressing uncertainty appear to be key in keeping knowledge building discourse going, in particular through linking new knowledge claims to previous claims and simultaneously inviting others to refine the contributed claim.