Cereal bran possesses enzymes and antinutrients which should be reduced for their usability as a food ingredient high in dietary fiber, minerals, and antioxidants. This study investigated the influence of high-intensity ultrasound on enzymatic and antioxidant activities, phytic acid content (PA), and functional and rheological properties of oat and barley bran. Ultrasonication was performed at three specific energies (87 kJ/kg, 217.5 kJ/kg, and 348 kJ/kg), without or with pulsation (5 s emission, 10 s pause). Bran was assessed for β-glucanase, phytase and antioxidant activities, PA and total phenolic content, hydration, and rheological properties. β-glucanase from oat bran was inactivated up to 82% and from barley bran up to 55%, depending on the ultrasound-specific energy and pulsation. PA and antioxidant activities were higher in native oat bran (PA 17.35 mg/g d.w., FRAP 6.96 µmol TE/g d.w., 3.30 µmol FAE/g d.w.) compared to barley bran (PA 11.53 mg/g d.w., FRAP 2.27 µmol TE/g d.w., 5.30 µmol FAE/g d.w.). In both bran types, phytase activity increased (40–44%) after treatments at 87 kJ/kg, and on average, PA was reduced by 17% in oat bran and by 39% in barley bran. Depending on the energy and pulsation, bran ultrasonication reduced total phenolic content (27–55%), antioxidant activity (by 28–48%), complex viscosity (62–71%), and maximum stress tolerance (46–68%) while increased water swelling (42–48%) and retention capacity (44–59%). Hence, high-intensity ultrasound is a useful technique in reducing antinutrients while altering the enzymatic activity and functional properties of the bran. These results could help the wider application of bran in food production.