Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you to remember:
Demons are mean. This particular demon bites you on the knee, creating an "O" shape on your knee. He tells you he has cursed you with the "O" knee (おに).
Hey! Stop this weak act and show us your true form, you demon!