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| O | Book and author | Other notes |
| Olia | The Castle of Tangled Magic by Sophie Anderson | |
| Olive | Olive, the Other Reindeer by J. Otto Seibold | |
| Oliver Ollie | Tales of Oliver Pig by Jean Van Leeuwen The Wish Factory by Chris Riddell Oliver and the Monsters by Tony Blundell Oliver and Patch by Claire Freedman Oliver and the Seawigs by Philip Reeve Oliver Moon books by Sue Mongredien Harry Potter books by J. K. Rowling A Dog Called Flow by Pippa Goodhart A Werewolf Named Oliver James by Nicholas John Frith Oliver Twist by Charles Dickens McTavish books by Meg Rosoff | S S H, ! |
| Olivia (see also Livvy) | Olivia by Ian Falconer Olivia the Orchid Fairy by Daisy Meadows The Balloon Cloud by Clare Hanlon Wonder by R.J. Palacio (known as Via) | S |
| Olympia | Olympia the Games Fairy | S |
| Omar | Everybody Feels Scared by Moira Butterfield Planet Omar: Accidental Trouble Magnet and Unexpected Super Spy by Zanib Mian Omar, the Bees and Me by Helen Mortimer Boy Giant, Son of Gulliver by Michael Morpurgo | |
| Omri | The Indian in the Cupboard books by Lynne Reid Banks | S |
| Oonagh | Finn McCool stories (trad.) | |
| Orla | Orla the Inventor Fairy by Daisy Meadows Orla and the Serpent’s Curse by Chris Haslam | S |
| Orlando | Orlando the Marmalade Cat by Kathleen Hale | S |
| Orville | Penguinaut! by Marcie Colleen | |
| Oscar | Oscar the Hungry Unicorn by Lou Carter That Pesky Rat by Lauren Child Friends by Rob Lewis | |
| Otto | Otto the Book Bear by Katie Cleminson | |
| Ottoline | Ottoline and the Yellow Cat by Chris Riddell | S |
| Owen | Owen by Kevin Henkes A Mummy for Owen by Marion Dane Bauer (based on a true story) Owen and the Soldier by Lisa Thompson | ! |
