Second language teaching has recently witnessed a transition from the conventional flipped classes to a fully online flipped instruction. The present study aims to examine the differential effects of two modes of fully online flipped instruction, i.e. interaction-embedded (IE) and interaction-reduced (IR), on EFL learners' speaking. Aligning with a quasi-experimental design, the current study recruited 43 intermediate learners from an English language institute, comprising an experimental group (n = 22) which received an interaction-embedded online flipped instruction with a student-centered teaching style (hereafter IE group) and a comparison group (n = 21) which underwent an interaction-reduced online flipped instruction following a teacher-dominated mode of teaching (hereafter IR group). The participants were selected from intact classes following a convenience sampling method and homogenized based on the Oxford Placement Test (OPT) scores. The results of Mann-Whitney U and Wilcoxon tests indicated that IE and IR modes of online flipped instruction both affected the students' speaking significantly. Further analysis confirmed the superiority of the IE group to the IR group; however, the observed effect size was negligible. The qualitative findings from semi-structured interviews revealed that the fully online flipped classes secure an innovative, effective and collaborative learning that helps overcome the disorientation problems and apprehension of synchronous live interactions. As such, we posit the significance of online flipped instruction as an approach that bolsters computer-mediated, collaborative speaking and collective scaffolding through establishing an online learning community.