This study explores how mentor teachers in specialized teaching areas, particularly chemistry, interact in an Online Professional Development (OPD) program. The Mentor Teacher Professional Development (MeT-PD) program was designed to improve mentoring practices by creating opportunities for collaborative learning through various online activities, such as Individual Response (IR), Interactive Individual Response (IIR), Small Group Discussions (SGD), and Large Group Discussions (LGD). Using a qualitative case study approach, the research analyzed data collected from Zoom recordings and Nearpod activity logs. The findings indicate that while LGDs were useful for interactions between facilitators and learners, they were not as effective in fostering interaction among learners themselves, mainly due to the cognitive demands and how these discussions were structured. On the other hand, SGDs seemed to foster stronger participant interaction, probably because the smaller group settings led to more valuable exchanges. These findings highlight the need of thoughtful planning of OPD activities, with particular focus on group size management and selection of suitable discussion formats to improve both interaction and learning outcomes.