ONCEIM is a collective free improvisation ensemble consisting of 30 musicians. Beyond free improvisation, the orchestra also performs new comprovisation works, commissioned from a variety of composers such as Eliane Radigue, Stephen O'Malley, John Tilbury, or Jean-Luc Guionnet. In this paper, we present a case study based on the 23 pieces commissioned by ONCEIM over a period of ten years, from 2012 to 2022. We first give an account of the different approaches encountered, illustrated by some examples of pieces. We then show how ONCEIM's musicians use re-notation strategies in the process of rehearsing such comprovisation pieces. Finally, we reflect on the role and use of electronics, as imagined by the composers with whom ONCEIM has collaborated, within a mostly acoustic setting.