Ariadne Oliver is a fictional character in the novels of Agatha Christie. She (like Christie) is a crime fiction novelist, the creator of the fictional Finnish detective Sven Hjerson, and a friend of Hercule Poirot.

Mrs Oliver often assists Poirot in his cases through her knowledge of the criminal mind. She often claims to be endowed with particular "feminine intuition", but it usually leads her astray. She is particularly fond of apples, which becomes a plot point in the novel Hallowe'en Party.

In the books, Oliver's most famous works are those featuring her Finnish detective Sven Hjerson. Since she knows nothing of Finland, Oliver frequently laments Hjerson's existence. In many of her appearances, Oliver – and her feelings toward Hjerson – reflect Agatha Christie's own frustrations as an author, particularly with the Belgian Hercule Poirot (an example of self-insertion). The self-caricature has also been used to discuss Christie's own follies in her earlier novels. For instance, in Mrs McGinty's Dead, Mrs Oliver talks of having made the blowpipe a foot long (30 cm) in one of her novels, whereas the actual length is something like four-and-a-half feet (1+1⁄2 yards (140 cm)) – the same mistake Christie made in Death in the Clouds.

In The Pale Horse, Mrs Oliver becomes acquainted with the Rev. and Mrs Dane Calthrop, who are friends of Miss Marple (The Moving Finger), thus establishing that Miss Marple and Hercule Poirot exist in the same world. In Cards on the Table, there is a reference to Mrs Oliver's book The Body in the Library; this title was used by Christie six years later, for a novel featuring Miss Marple. Books by Ariadne Oliver and by a number of other fictitious mystery writers are discussed by characters in The Clocks (1963). Like Christie, she is a member of the Detection Club. Christie even thought of setting a murder at the Club with Oliver being one of the suspects as well as the detective, but it came to nothing, although in Cards on the Table, Mrs Oliver plays detective in a Poirot mystery involving a murder during a bridge game. A family crisis for Oliver's goddaughter Celia provides the plot in Elephants Can Remember.

Although Ariadne Oliver is consistently referred to as "Mrs Oliver", nothing is known about her husband. An offhand reference to her marriage is made in Elephants Can Remember.

In a short piece in John Bull magazine in 1956, Christie was quoted as saying, "I never take my stories from real life, but the character of Ariadne Oliver does have a strong dash of myself." The author of the article went on to state, "It is perfectly true that sometimes she works at her stories in a large old-fashioned bath, eating apples and depositing the cores on the wide mahogany surround."

Ariadne Oliver does not function as a detective, even in the novel in which she appears without Poirot (The Pale Horse). In Cards on the Table, she does interview some of the suspects, which in turn allows her to discover a hidden motive that even the police were unable to find; in Elephants Can Remember, she again interviews witnesses, but none of the essential ones. On the surface, Christie appears to use Oliver mainly for comic relief or to provide a deus ex machina through her intuitive or sudden insights, as in Third Girl, in which she furnishes Poirot with virtually every important clue, or in The Pale Horse, where she inadvertently tips the investigators off about the type of poison used to kill the murder victims.

However, beneath the surface of the stories, Ariadne Oliver is in fact Christie's tool for getting hints and clues to the reader. In Dead Man's Folly, Oliver creates a murder hunt and specifically tells Poirot that she has hidden "six clues" to the solution in the game. In fact, Christie inserts six clues for savvy readers throughout the novel. In The Pale Horse, Oliver states that she only writes about very "plain" murders, not anything supernatural -- "'just about people who want other people out of the way and try to be clever about it'." Of course, the solution to the murders is exactly that and not supernatural at all.