Due to the challenges pertaining to online professional development (OPD), there has been a recent shift in OPD from the one-shot online workshops, trainings and webinars grounded in traditional professional development models (Prestridge & Tondeur, 2015) to the online professional learning communities designed through constructivist pedagogies (Whitehouse, McCloskey, & Ketelhut, 2010). Addressing this change in the OPD paradigm, this qualitative study set out to investigate the impact of online lesson study as an unexplored type of OPD on a group of Turkish EFL teachers' development. In the 2017-2018 fall term, 4 EFL teachers from different schools volunteered to engage in an online lesson study procedure for 13 weeks. For online lesson study procedure, Dudley's (2015) version of Lesson Study (LS) was adapted to the online medium with the inclusion of some synchronous and asynchronous tools. Within that procedure, teachers worked online collaboratively to set goals for student learning and co-planned research lessons that target these goals. One of the teachers taught the lesson in his/her own classroom and other teachers watched the video recording of the taught lesson. In an online post-lesson meeting, they reflected on the data to improve the lesson and taught the revised lesson in the same or a different teacher's classroom (Lewis, 2009). Within the scope of this study, the participating teachers completed this process three times which constituted three LS procedures in total. In addition, the teachers also participated in six webinars related to English language teaching as part of a Marie Curie Project. The video-recordings of prelesson and post-lesson discussion meetings, video-recordings of the taught lessons, lesson plans, teacher posts in online platforms and teacher interviews composed the data collection tools. But for the sake of this study, interview data from the first LS cycle are reported here. The results of the preliminary analysis show that an online professional development program that included a combination of webinars and online lesson study procedure led to perceived cognitive changes in a group of Turkish EFL teachers. These changes were concerned with increased technological knowledge, increased self-appraisal and self-reflection, development of language proficiency and increased knowledge of instructional strategies.