OM# (om-sharp) is a computer-assisted composition environment derived from OpenMusic: a visual programming language dedicated to musical structure generation and processing. The visual language is based on Common Lisp, and allows to create programs that are interpreted in this language. Visual programs are made by assembling and connecting icons representing Lisp functions and data structures, built-in control structures (e.g. loops), and other program constructs. The visual language can therefore be used for general-purpose programming, and reuse any existing Common Lisp code. A set of in-built tools and external libraries make it a powerful environment for music composition: various classes implementing musical structures are provided, associated with graphical editors including common music notation, MIDI, OSC, 2D/3D curves, and audio buffers.