Objectives: In 2013, the World Health Organization (WHO) recommended new criteria for diagnosing gestational diabetes mellitus (GDM). Our study aimed to compare the incidence of GDM and its complications among Omani pregnant women using the new diagnostic criteria compared to previous criteria published in 1999. Methods: This retrospective cohort study was conducted between January and December 2016 at the Bawshar Specialized Polyclinic in Muscat, Oman. A total of 613 Omani women were included in the study. Baseline maternal characteristics such as age, body mass index, parity, and socioeconomic status were recorded in addition to maternal and neonatal outcomes. Results: Based on the current diagnostic criteria, the incidence of GDM was 48.5% (n = 297); however, this dropped to 26.4% (n = 162) when applying the old criteria. Moreover, rates of maternal complications including polyhydramnios (5.6% vs. 4.6%), pregnancy-induced hypertension (3.2% vs. 1.5%), and preterm delivery (3.2% vs. 1.5%) were slightly higher among women with GDM diagnosed using the latest criteria. However, these differences were not statistically significant. Neonatal complications were also slightly more frequent among the first group, without any significant differences. Conclusions: The incidence of GDM among Omani women rose dramatically when utilizing the latest WHO diagnostic criteria, owing to a lower fasting blood glucose cut-off value. In addition, a comparison of the frequencies of maternal and neonatal complications supports the validity of the new criteria. These findings should be taken into consideration by decision-makers in Oman when planning antenatal and postnatal services.