He took his first degree at the Liege Conservatoire, where he obtained First Prize for saxophone. He then took classes with the likes of John Ruocco, Steve Coleman and Joe Lovano. Around the mid-Eighties he founded his first band, Trio Bravo (later renamed Trio Grande), with Michel Massot and Michel Debrulle. Roundabout the same time he meets drummer Stéphane Galland and bass player Michel Hatzigeorgiou in Brussels. In 1991 the three of them go to Central Africa, where they meet the Akapy pygmies. Upon their return they found the now famous trio Aka Moon.
Cassol often performs with artists such as Fredéric Rzewski, Garrett List, David Linx, Kris Defoort, Toots Thielemans and DJ Grazzhoppa. He has also taken up composing for the music theatre and dance and composed for Anne Teresa De Keersmaeker and Alain Platel's Les Ballets C de la B (2006), among others.
In 1995 he composed the album 'Variations On A Love Supreme' with Kris Defoort. Together, they gave a much talked about concert at Jazz Middelheim. In 1998 he won the Django D'Or for best francophone jazz artist. Since September 2001 he has also been using the aulochrome, a new instrument developed by François Louis. It was introduced to the public in Paris in October 2002. Since 2006 he has often been heard with Malinese singer Oumou Sangare.
He currently lives in Brussels and teaches at the Etterbeek music academy.