Oxygen was discovered independently by Joseph Priestley in England and Carl Scheele in Sweden. The existence of such a gas as oxygen had been theorized as long ago as the time of Leonardo da Vinci. Others had isolated oxygen before Priestly and Scheele but didn't recognize it.
Priestly discovered oxygen on Aug. 1 while in the employ of the Earl of Shelburne in Wiltshire, England. He discovered it through heating mercury oxide or lead.
Carl Wilhelm Scheele discovered oxygen around the same time as Priestly, but because Priestly published his findings first, he is given the credit for the discovery.