[1] His father was a member of the Prague National Theatre Orchestra, and his younger brother Miroslav was a choirmaster.
[1] After finishing his studies at the gymnasium, he enrolled at the Academy of Performing Arts in Prague where he was a pupil of Karel Ančerl.
In 1949 Košler joined the Olomouc Opera, where he conducted works by Leoš Janáček (The Makropulos Affair) and by W. A. Mozart (Così fan tutte, The Marriage of Figaro).
In 1963 he won the respected Dimitri Mitropoulos Conducting Competition in New York City, together with Claudio Abbado, and the Argentinian Pedro Ignacio Calderón, after which he became assistant conductor to Leonard Bernstein at the New York Philharmonic for one year.
[2] Košler made many recordings, including a group of works by Mozart, Dvořák, and Tchaikovsky in Barking Town Hall with the London Philharmonic Orchestra.