In which Agatha Christie novel does the character Ariadne Oliver first appear?

Ariadne Oliver first appears in the novel Cards on the Table. Published in 1936, the book introduces Oliver as a successful crime novelist who joins Hercule Poirot, Superintendent Battle and Colonel Race at an unusual bridge party hosted by the sinister Mr Shaitana.

Oliver is widely regarded as Agatha Christie's humorous literary alter ego. Like Christie, she writes popular detective fiction, complains about the demands of readers and publishers, and occasionally pokes fun at the conventions of the genre. Oliver's fictional detective, Sven Hjerson, is also a playful reflection of Christie's own experience creating recurring detectives.

There is one important distinction behind this question. Oliver appeared in print earlier in the short story The Case of the Discontented Soldier, published in the Parker Pyne Investigates collection. Cards on the Table is nevertheless the accepted answer when asking for her first appearance in a full-length novel or her first appearance alongside Poirot. She later returned in novels including Mrs McGinty's Dead, Dead Man's Folly, Third Girl, Hallowe'en Party and Elephants Can Remember.