Open online courses are becoming more prevalent at local level and for professional development objectives. Proper instructional design combined with use of online tools can promote learner interaction in online environments. Using the Community of Inquiry (CoI) framework, this study aimed at examining learners' interaction and their perceptions of teaching presence, social presence, and cognitive presence in an open online course offered for professional development in three Swedish universities. The course was free and open to all, attracting participants from all over the world. In order to understand the online interactions of the course, three presences of CoI were matched to three types of interaction (Moore, 1989). Data were collected through a slightly revised version of the CoI instrument and open-ended questions were added. Survey results showed that participants had high perceptions of the three presences in the course. Results also yielded significant relationships between teaching presence and cognitive presence, as well as social presence and cognitive presence. The findings suggest that deploying a set of online tools combined with appropriate pedagogical approaches in designing open online courses could foster learner interaction especially learner-content interaction and cognitive presence.