A scanning tunneling microscope (STM) was used to control the pathway of the dissociation of single O2 molecules chemisorbed on Ag(110) at 13K. Tunneling of electrons from the STM tip into the O2 caused dissociation of the molecule, giving rise to two adsorbed O atoms separated along the [11¯0] direction. In contrast, the ejection of electrons from the O2 molecule produced adsorbed O atoms separated along the [001] direction. These results illustrate that control of the dissociation pathway and product formation are associated with a specific molecular orbital located at the Fermi level.