Online collaborative learning has typically been studied within the context of learning communities. Little is known about the potential influence of students' personalities on online communication, group interaction, and task engagement among members of a learning community. This study used a mixed-method, triangulation design, involving the collection and analysis of both quantitative and qualitative data, to investigate the effects of personality on communication type and pattern, message length, task engagement, and student attitude toward online learning. Seventy students were organized into four personality-profile groups based on their Five Factor Personality Test scores, for the discussion of assigned case studies. Discussion messages were analyzed using Logistical Regressions for communication type and pattern, ANOVAs for message length, and Z-tests for pairwise comparisons for task engagement. The results indicate that personality affects communication type, pattern and task engagement but not message length. Students' attitudes toward online discussion were generally positive. The results provide guidelines for forming groups and designing activities for online collaborative learning.