Saori Ōnishi is a Japanese voice actress born on August 6, 1992, in Chiba Prefecture, Japan. She is affiliated with the talent agency I'm Enterprise. Her decision to pursue a career in voice acting was inspired by watching anime in elementary school, with voice actresses Maaya Sakamoto and Sayaka Kobayashi cited as her inspirations. During her school years, she was a member of her brass band club. She trained at the Nippon Narration Engi Kenkyūjo (Japan Narration Acting Institute) starting in 2008 before joining I'm Enterprise in 2012, the same year she made her professional debut.

Ōnishi's first role after joining her agency was in a 2012 drama CD. Her first lead role in a television anime came in 2014 as Jamie Hazaford in Argevollen, a moment she has recalled as being so emotional that she cried while walking down the street upon learning she had been cast. The year 2014 also saw her take on significant roles such as La Folia Rihavein in Strike the Blood and Fianna Ray Ordesia in Blade Dance of the Elementalers.

A significant career milestone was her performance as Eriri Spencer Sawamura in the popular series Saekano: How to Raise a Boring Girlfriend, which premiered in 2015. That same year, she began voicing two other prominent and long-running characters: Hisako Arato in Food Wars! Shokugeki no Soma and Ais Wallenstein in Is It Wrong to Try to Pick Up Girls in a Dungeon?. She has continued to voice these characters across multiple subsequent seasons and OVAs. Other notable roles from this period include Nine-Tails Miyuki in Shomin Sample and Hiyori Jūjō in Katana Maidens: Toji no Miko.

In 2018, Ōnishi's performance was recognized with the 12th Seiyu Awards for Best Supporting Actress and Best Personality Award, sharing the latter with fellow voice actress Ayane Sakura for their work on the radio program Saori to Ayane no "To Shitai" (想和佐仓做的大西). The late 2010s saw her continue to build an extensive filmography with roles like Beelzebub in As Miss Beelzebub Likes It., Muramasa Senju in Eromanga Sensei, and Mejiro McQueen in the massively popular Uma Musume Pretty Derby franchise.

Her career has remained active into the 2020s, taking on a wide variety of characters. These include Kaori Shirasaki in Arifureta: From Commonplace to World's Strongest, Chateau Dankworth in Love of Kill, Shikimori in Shikimori's Not Just a Cutie, Akari Watanabe in More Than a Married Couple, But Not Lovers., and Runrun in Magical Circle Guru-Guru. In July 2023, it was announced that Ōnishi would be temporarily reducing her work activities due to health concerns. She has continued to voice roles in subsequent years, such as Dahlia Rossetti in Dahlia in Bloom and Tsubaki Kadomori in Malevolent Spirits: Mononogatari..

Throughout her career, Ōnishi has frequently collaborated with a consistent group of voice actors and production teams. She has worked extensively with colleagues such as Ayane Sakura, Inori Minase, Yui Ogura, and Yoshitsugu Matsuoka across numerous series, including Saekano, DanMachi, Eromanga Sensei, and The Detective Is Already Dead. In addition to her anime work, she is also an active radio personality and has contributed theme song performances for several of the series in which she stars, such as Comic Girls, Katana Maidens, and The Detective Is Already Dead.