Diabetes mellitus presents a great diversity of treatments that cause adverse effects; therefore, plants are a source of compounds that may have fewer adverse effects; Cinnamomum cassia (C. cassia) has compounds with potential antidiabetic activity. The objective was to evaluate the antidiabetic effect of C. cassia oil (CCO) and its impact on oxidative stress in Wistar rats. Five groups were evaluated: (1) sham (SH), (2) 300 mg/kg CCO (CCO), (3) diabetic (D) induced with alloxan, (4) D + 300 mg/kg of CCO (D + CCO), and (5) D + 500 mg/kg of metformin (D + MET); all were treated for 5 days. CCO did not show alteration in aspartate aminotransferase (AST) and alanine aminotransferase (ALT) vs. SH. D + CCO vs. D significantly reduced glucose (333 ± 109 vs. 458 ± 81 mg/dL), ALT (66 ± 15 vs. 160 ± 54 U/L), AST (119 ± 26 vs. 243 ± 104 U/L), and blood urea nitrogen (18.8 ± 2.3 vs. 29.2 ± 6.9 mg/dL). No significant changes were observed in D + CCO vs. D in malondialdehyde (MDA), reduced glutathione (GSH), and superoxide dismutase (SOD), whereas a significant reduction in MDA and GSH was achieved in D + MET, with an increase in SOD. There was a reduction in Rela and Gpx in D + CCO and D + MET vs. D. CCO has antidiabetic and anti-inflammatory effects and reduces ALT, AST, and BUN levels.