The widespread of online education is clear and the demand for online education is increasing rapidly, so the quality of offered online courses is urging. During COVID-19 restrictions, unprepared institutions, instructors, and students for online education were clear, especially in STEM. So there is a need for more research on online mathematics education to improve the quality of offered online courses. This study examined online mathematical discourse to find out ways an instructor of an online developmental undergraduate-level mathematics course plan for and facilitates mathematical discourse, the instructor's perceptions of the learning opportunities that are afforded to students through his planning and facilitating of online mathematical discourse, and the instructor's perceptions of the major challenges to planning and facilitating mathematical discourse. A qualitative case study approach was utilized. Interviews and observation were employed to collect data. One community college associate mathematics professor in the southwestern United States with his developmental online mathematics course "College Algebra" was recruited for this study. Categorizing thinking procedure, as described by Marshall and Rossman (2016) was used to analyze data. Analyzing data resulted in recognizing three themes, 1) the instructor's view of himself as a mathematics instructor in face-to-face and online settings, 2) planning and providing opportunities for discourse to happen, and 3) students, instructor, and institution-related challenges for planning and facilitating discourse. The main findings are 1) planning and facilitating online mathematical discourse can be through course formats, course formats could present opportunities for discourse to happen and motivate students to participate in that discourse, 2) three learning opportunities were observed, commitment to mathematical language, students participated and asked their questions during the synchronous sessions, and there was some opportunity for shared authority in the classroom, and 3) perceived challenges for planning and facilitating discourse are related to students, instructor, and institution. Implications of findings are for practice and future research. The main implication for practice, mathematics instructors should train themselves about carrying out meaningful mathematical discourse through engaging in professional development, and future research is to focus on the impact of professional development on carrying out mathematical discourse. Keywords: Discourse, Synchronous, Planning, Facilitating, Opportunities, Challenges. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]