Oscar Loew (2 April 1844 – 26 January 1941) was a German agricultural chemist, active in Germany, the United States, and Japan in the late 19th and early 20th centuries.

[1] Loew was an assistant in plant physiology at the City College of New York and participated in four expeditions to the southwestern United States in 1882 before returning to Munich, Germany, where he collaborated with Carl Nägeli.

Loew served as instructor at Tokyo Imperial University between 1893 and 1907, succeeding Oskar Kellner as professor of agricultural chemistry there.

Loew invented a method to produce formaldehyde from methanol by oxidation with atmospheric oxygen and metallic copper as a catalyst.

[2] In 1892 Loew observed that both calcium and magnesium can be toxic to plants when there is an excess of one and a deficiency of the other, thus suggesting there may be an optimal Ca:Mg ratio.