Orac is a classical molecular dynamics computer program (written in FORTRAN language) for simulating complex molecular systems at the atomistic level. The code was written originally in 1989-90 by Massimo Marchi during his stay at IBM, Kingston (USA). The code was further developed at CECAM in 1995 and released under the GPL license in 1997. The latest release of Orac may be run in parallel using the standard Message Passing Interface libraries, allowing replica exchange simulations, multiple walkers metadynamics simulations and multiple steered molecular dynamics nonequilibrium trajectories.