Books ordered by title: O
| Author | Title (Number of quotes) |
| Neil Gaiman | Odd and the Frost Giants (3) |
| Stephen King | On Writing (5) |
| David Nicholls | One Day (3) |
| István Örkény | One Minute Stories (2) |
| Max Tegmark | Our Mathematical Universe (2 + 2 quoted) |
| Albert Einstein | Out of My Later Years (4) |
| Malcolm Gladwell | Outliers (2) |
| H. Gilbert Welch | Overdiagnosed (2) |