Aims: Murine studies show a promising effect of high-fiber β-glucan on glycemic control and serum lipids. In addition, β-glucan has recently been found to have strong antioxidant and immunomodulatory effects. Oat flakes are a natural source of β-glucan. However, the effects of oat flakes on glycemic variability, dyslipidemia, and pancreatic duodenum homeobox-1 (PDX-1) levels in type 1 diabetes (T1D) remain unclear. Hence, this study assessed the effect of oat flakes on glycemic variability, dyslipidemia, and PDX-1 among adolescents with T1D. Materials and Methods: Sixty adolescents with T1D were divided into 2 equally matched groups. Group A received oat flakes β-glucan 6 g per day for 3 months in addition to an ordinary diet and insulin regimen. Group B received an ordinary diet and insulin regimen. This was followed by crossing over both arms for another 3 months after a two-week washout period. All participants underwent auxological assessment, continuous glucose monitoring (CGM), hemoglobin A1c (HbA1c), fasting lipids, and PDX-1 measurements at baseline, 3 months, and 6 months. Results: Oat flakes consumption resulted in a significant decrease in the coefficient of variation, HbA1c, serum cholesterol, triglycerides, and LDL-C levels (p < 0.001), with a significant increase in TIR, HDL-C, and PDX-1 levels (p < 0.001). However, all these effects waned after the stoppage of the oat flakes, except for HDL-C. Conclusions: Oat flakes have a favorable outcome on glycemic metrics, lipid profile, and PDX-1 in adolescents with T1D.