Chester Barnard was an American manager and public administrator who greatly influenced economics and the history of economic thought. Oliver Williamson’s early works were published after Barnard’s death, but the intellectual proximity they show to Herbert Simon, along with the significant influence of John Commons on their analysis, brings them closer. This study investigates Barnard legacy (particularly his work in 1938) in Williamson’s “science of organisation” project by analysing the Barnardian origins of his organisational micro-approach from their complementary theories of cooperation.