Massive Open Online Courses (MOOCs) are delivered through dedicated platforms that provide educational opportunities to a large number of learners around the globe. The discussion forum is a key part of a MOOC platform. Structured communication between learners or between learners and instructors can take place through the forum. This communication has been shown that can have strong learning impact. Teaching Assistants (TAs) have a crucial role coordinating and supporting learners within a MOOC forum. In this paper, we investigate the impact a forum design can have on the TA’s effectiveness while supporting the learners of a MOOC. Towards this objective, a mixed-methods study was performed on two MOOCs delivered through the OpenEdX platform. The goal was to reveal any design issues initially through a participatory ethnographic study and complementarily through a formal usability evaluation. Moreover, through interviews with the TAs, problems they faced while supporting learners were confirmed. The results of this study indicate that the OpenEdX forum design faces a variety of issues that need to be considered by course designers. Such issues cause various problems to teaching assistants, hindering effective support to learners and therefore affecting the learners’ experience. It is further expected that the findings of this study may contribute to effective re-design of MOOC platform forums, more effective and efficient TA interventions and ultimately to improved learning.