This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means cover, so the verb version is to cover or to hide.

Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn this reading yet, so here's a mnemonic to help you:

You decide to cover yourself with a sheet because you want to pretend to be a ghost. But covering yourself isn't enough. If you really want people to believe in your disguise you have to say, "おお!" Otherwise they'll think you're a covered chair or a bush. If you おお at them they'll definitely know you're a ghost covered in that sheet.