Flow past a transversely oscillating circular cylinder in a channel can be used as an active mesoscale mixer, kinematics of which was investigated by Celik ["Flow past an oscillating circular cylinder in a channel with an upstream splitter plate," Phys. Fluids 20, 103603 (2008)] at Re=100. This study presents numerical simulations of species transport in the mixer, obtained for various cylinder excitation frequencies and the species inlet configurations for a wide range of Peclet numbers. Mixing indices are calculated on the vortex spacing based mixing blocks, which is a newly introduced concept that utilizes periodicity of the vorticity field. Mixing index comparisons show that mixing efficiency is strongly dependent on the identity of the species within wall shear layers and vortex cores. For the cylinder excitation frequency of 25% higher than the natural vortex shedding frequency, 60% and 46% mixing enhancements relative to the straight channel and the stationary cylinder cases are observed at Pe=800, respectively.