Oxygen chemisorption at metal surfaces has been shown through a combination of scanning tunnelling microscopy and photoelectron spectroscopy to involve transient states that provide low energy pathways for a wide range of surface reactions including the catalytic oxidation of ammonia and hydrocarbons. The kinetically 'hot' transients are disordered and mobile, become unreactive when they form ordered structures, and are characterized by non-classical kinetic behaviour. The role of surface additives (caesium) in controlling oxygen structures and the implications of oxygen transients for theory and reaction mechanisms in applied catalysis are considered.