The use of effective oral questioning in the teaching of mathematics can stimulate students' thinking and encourage them to think critically. As a result, this study was carried out to identify the oral questions used by teachers when teaching mathematics. This can encourage students' critical thinking. This was a qualitative study in the form of a case study conducted in six schools in a Malaysian state with a total of six study participants. They were chosen on purpose based on the specific criteria set by the researcher. The data was collected using observation methods, interviews, and field notes to gain an in-depth picture of the phenomena studied. The study findings were analyzed using the continuous comparison method to identify the themes and subthemes involved. The researchers used several methods to improve the validity and reliability of this study, including triangulation, the consent of the study participants, peer reviews, audit trails, researcher bias, and a long period in the field. According to the study findings, mathematics teachers frequently use three types of oral questions to encourage their students to think critically: prompting questions, reflective questions, and clarification questions. These types of oral questions are effective at encouraging students to think critically when trying to solve mathematical problems. This study implies that teachers should use caution when asking oral questions so that the students' thinking is stimulated, rather than focusing on memorizing important mathematical algorithms and procedures.