Cassia O'Reilly, better known by her stage name Cosha (formerly Bonzai), is an Irish singer and songwriter currently based in London.

Born in Indiana, US to a jazz singer mother and raised in Wicklow, Ireland by her drummer father, she moved to London at 17 to start a degree at Tech Music School. She found a job as a backing singer for rising star NAO, whose management introduced her to ​Mura Masa. Cosha went on to release her first two EPs on his independent label, Anchor Point under the alias Bonzai.

In 2018, she signed to Columbia Records and released her first mixtape R.I.P Bonzai, featuring production from ​Mura Masa, Jim-E Stack, Danny L Harle, and Rostam, among others. With the mixtape’s release, she started releasing music as Cosha, the phonetic pronunciation of her first name. In 2020, she started releasing singles under her own label, Ashtown Lane.