Ellis was born on February 8, 1928, in Ffynnongroew, Flintshire, Wales. His mother was an amateur harpist and encouraged him to learn the instrument. He studied with Alwena Roberts before attending the Royal Academy of Music, where his teacher was Gwendolen Mason. In 1959, Ellis succeeded Mason as the harp professor at the Royal Academy. His stay with the London Symphony began in 1961, shortly after he first began his involvement with Britten. He was a creative player during his London Symphony days and was even known to toss in his own solos while performing with the orchestra. When the Melos Ensemble was formed in London in the late '60s, Ellis was a natural choice to become the ensemble's harpist. The ensemble was dedicated to performing the often neglected repertoire for extended chamber ensemble, in the case of the harp, leading to the impressionist masterpieces of Ravel and Debussy.
Britten's Suite for Harp was composed in 1969 after Ellis asked the composer for a solo piece, and it was premiered during that year's Aldeburgh Festival. Ellis toured extensively in America and Europe with tenor Sir Peter Pears from 1973 to 1980, prompting eager composers to submit songs to the duo. That included Britten, who came up with Canticle V and A Birthday Hansel during this period, the latter piece a special request from the Queen. After this, Ellis began to be referred to as "the Queen's harpist." Ellis continued this tradition, performing with his son, Tomos, who was also a tenor, in the U.K. and beyond. Ellis and Britten also collaborated closely on the 1976 Eight Folk Song Arrangements, with Ellis creating virtuoso harp accompaniment as well as English lyrics for Welsh text.
Ellis remained at the Royal Academy until 1989, and he retired from the London Symphony in 1994. He was a constant champion of his instrument, conducting multi-harp workshops and leading the Osian Ellis Harp Ensemble. In 1999, he conducted a 12-harp ensemble at a special concert for the Florida International Festival, where he was photographed practicing harp on the beach. He also had compositions written for him by Gian Carlo Menotti, William Schuman, and Alun Hoddinott, among others. He frequently performed excerpts from the harp compositions by composer Jørgen Jersild, many of which were dedicated to Ellis. Following a superb academic career, he was made an Honorary Fellow by the University of Wales in 2000. His short book, The History of the Harp in Wales, was published by the University of Wales Press. He was the subject of several full-length documentaries produced by British and Welsh television, the latter filmed when he was beginning his career. Ellis retired from performing to care for his wife Rene, a violist to whom he was married for over 60 years. They had two sons together, Richard and Tomos; Tomos died in 2009 and Rene in 2012. The Wales International Harp Festival in Caernarfon held a concert to honor Osian's 90th birthday in 2018. The celebration inspired Ellis to return to playing the harp. He also returned to composing, which resulted in a Lachrymae for solo harp and Cylch o Alawon Gwerin Cymru ("A Circle of Welsh Folk Songs"), written for bass-baritone Bryn Terfel and harpist Hannah Stone. Ellis died on January 5, 2021, at the age of 92. ~ Eugene Chadbourne & Keith Finke, Rovi