I am not sure whether Sargent owns the highest production in oil painting. But the quantity of his works is impressive to me.
John Singer Sargent (January 12, 1856 – April 14, 1925) was an American artist, considered the "leading portrait painter of his generation" for his evocations of Edwardian era luxury. During his career, he created roughly 900 oil paintings and more than 2,000 watercolors, as well as countless sketches and charcoal drawings.