Olaf Wilhelm Isaachsen (May 16, 1835 – September 22, 1893) was a Norwegian landscape and genre painter.
He was born in Mandal in Vest-Agder county Norway. Isaachsen belonged to a branch of an affluent and educated merchant family from in Kristiansand. He was the son of jurist Daniel Peter Christian Isaachsen, grandson of politician Isaach Isaachsen and great-grandson of (1744-1813), a Norwegian shipbuilder . He was the uncle of physicist Daniel Isaachsen, scientist Haakon Isaachsen (1867–1936), painter Herman Willoch and naval officer Odd Isaachsen Willoch.
He attended Nissens Latin School in Christiania (now Oslo). He later studied under and Johannes Flintoe at the Royal School of Drawing from 1850, in Düsseldorf from 1854 to 1859, and in Paris under Thomas Couture (1859-1860) and Gustave Courbet (1861-1862). After a visit to Italy he returned home in 1864 with a view to becoming an artist with a strong focus on archeology and history. He ultimately settled in Kristiansand, although with frequent periods spent abroad.
His artistic style reflected naturalism and in later years impressionism. Isaachsen was regarded as one of Norway’s more prominent artists and great colourist with motives of the coast landscape of Southern Norway. He also painted a wide range of portraits. Moreover, he found a full valid artistic expression in Setesdalen. His art often reflected the valley's distinctive culture. He is represented with 24 works in the National Museum in Oslo (NM), former National Gallery of Norway. The Regional Museum in Kristiansand, Sørlandets Kunstmuseum (SKMU) has 58 works.