Dissociation of a highly excited diatomic molecule in the Ar + Ar…$O_2$ and Ar + $O_2$ collisions is studied using trajectory dynamics procedures in the collision energy range of 0.050 to 1.0 eV. Between 0.050 and 0.2 eV, dissociation probabilities are very large for the complexed system compared to the uncomplexed system. This efficient dissociation of $O_2$ in Ar…$O_2$ is attributed to the ready flow of energy from the incident atom to the large-amplitude vibrational motion of the excited O2 via the van der Waals bond. Thermal-averaged dissociation probabilites of $O_2$ in Ar + Ar…$O_2$ near room temperature are nearly two orders of magnitude larger than those of $O_2$ in Ar + $O_2$.