Background and objectives: Phenolic compounds present in oats provide an opportunity for oats to increase its use in the form of functional food. In this study, eight varieties of Australian-grown whole-grain oat were investigated for their phenolic composition and antioxidant potential. Ultra-high-performance liquid chromatography–online 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), coupled with liquid chromatography quadrupole time-of-flight, was used to identify and quantify individual phenolic compounds and their respective antioxidant activity. Findings: Considerable variation in phenolic content (19.51 ± 1.20–52.49 ± 1.70 mg 100 g−1 GAE) and antioxidant activity was observed among the oat varieties. Several avenanthramides (AVNs) were detected with AVN A and AVN C being the most abundant. AVN C exhibited the highest antioxidant activity followed by caffeic acid, quercetin-3,4′-O-di-beta-glucopyranoside derivative, and 6-hydroxykaempferol 3,5,7,4′-tetramethyl ether 6-rhamnoside. Antioxidant activity in AVNs was observed to be the highest in AVN C followed by AVN B and AVN A. Conclusions: High levels of AVN C and the range of phenolic compounds present show promise of Australian-grown oats as an important source of antioxidant compounds. These antioxidant compounds may contribute to consumers' health benefits. Significance and novelty: This study contributes to the use of oats as potential functional food by characterizing its individual phenolic compounds and their antioxidant activity.