This study explores the nature of co-constructed interaction in group oral tests by examining whether a test-taker's own and his or her group members' extraversion levels and oral proficiency levels have different influences on conversational styles between two group sizes: groups of three and groups of four. Data were collected from 269 Japanese upper-secondary school students, who took group oral tests either in groups of three or four. All sessions were video-taped and transcribed following Conversation Analysis (CA) conventions. The data were quantitatively analysed in terms of goal-orientation, interactional contingency and quantitative dominance. Then, CA methodology was used to interpret and elaborate the statistical results. The findings have implications for our understanding of the group oral test construct and for appropriate choices of group size in group oral testing.