A new differential mobility classifier will be described. The instrument classifies aerosol particles in a channel flow between porous (or screen) electrodes. The aerosol enters the channel parallel to the porous electrodes, while a larger, particle-free cross-flow enters through one of the porous electrode. A potential difference between electrodes causes the charged aerosol particles to migrate upstream against the cross-flow. Only particles whose upward migration velocity balances the cross flow will be transmitted along the path of the classifier. Simulations of the OMAC show that it should give the same resolution at the traditional differential mobility analyzer (DMA) but with much lower voltages required for an equivalent resolution. This extended range of operation enables development of nanoparticle size distribution measurements with the same dynamic ranges as the DMA but in a much smaller package. This lower voltage also enables operation of the OMAC at lower pressure without the loss of dynamic range that traditions DMAs suffer and at higher peak resolutions than are possible with the DMAs.