On Agatha Christie's Birthday, a Google Doodle Whodunit

On the 120th anniversary of Agatha Christie's birthday, the Google Doodlers have struck again - this time adorning Google pages in the United Kingdom and Ireland with the following tease: "Doodle me this. Who killed 'O'?"

Personally speaking, I'm quite lousy at deciphering  these sorts of riddles. But I'm sure more than a few of you are regular crackerjacks, so leave your best guesses in the talkback section below. In the meantime, here's a very happy birthday to the grande dame of mystery fiction.