The increasing prevalence of diabetes mellitus (DM) worldwide represents a global public health emergency. Healthcare professionals play a major role in educating patients regarding important self-care and diabetes management strategies which can help prevent long-term complications. This study aimed to assess knowledge and practice regarding diabetes self-management among patients with type 1 and type 2 DM attending a diabetes education clinic in Oman. In addition, the study aimed to compare levels of knowledge and practice between new referrals and follow-up patients who had received more than two educational sessions. This cross-sectional study was conducted in a tertiary care institution in Muscat, Oman. A total of 181 sequential adult patients who attended the outpatient diabetes clinic between March and October 2018 were included in the study. Participants were categorized into new referrals who had not yet received diabetes education, and follow-up patients who had received more than two educational sessions from the diabetes educator. A previously validated self-administered survey was used to assess levels of knowledge and practice regarding diabetes self-management in the two groups. Of the 181 participants, 61 (33.7%) were newly referred, and 120 (66.3%) were follow-up patients. The mean age was 33.3 ± 12.9 years and 55.8% were female. Overall, 64.1% of the cohort demonstrated excellent knowledge regarding diabetes self-management, while 23.2% and 12.7% had moderate and poor levels of knowledge, respectively. Moreover, 73.5% exhibited satisfactory practices, while inadequate practices were reported by 26.5%. Overall, the diabetes education clinic was effective, with statistically significant differences in knowledge and practice levels noted between the two groups (p = 0.01 and 0.002, respectively). While the whole cohort demonstrated acceptable overall levels of diabetes self-management knowledge and practice, patients who had attended a structured diabetes education program demonstrated substantial improvements in knowledge and behavior compared to the new referrals. As such, specialized diabetes education programs are recommended to help reduce the incidence of acute and chronic complications of DM, thereby reducing the burden of this disease in Oman.