There is a big bull on the farm.
The bull has impressive horns.
Standard kanji spelling using 雄 (male) + 牛 (cow).
Alternate kanji spelling using 牡 (male animal). Less common than 雄牛 but still used.
Compound of 雄 (お, male) + 牛 (うし, cow). The reading おうし is a straightforward combination of the on'yomi of 雄 and the kun'yomi of 牛.