Significance This study demonstrates how the controlled deposition of isolated single molecules of the heterogeneous oxygen evolution reaction electrocatalyst cobalt oxide on a carbon nanoelectrode can be carried out and characterized by electrochemical imaging to obtain size and relative catalytic activity compared to larger clusters of the same material. Previous results for Pt atoms on Bi or Pb substrates for the hydrogen evolution reaction showed that larger-size clusters display a higher kinetic rate. In contrast, results in this work with cobalt oxides on carbon show that a single molecule, Co ₁ O _x , is a better catalyst than larger clusters ( n > 1).