Oscar Theander Harstad (May 24, 1892 – November 14, 1985) was an American Major League Baseball pitcher who appeared in 32 games for the Cleveland Indians during the 1915 Cleveland Indians season. After his baseball career, "Theander" as he was called by his family and friends, became a dentist serving the Milton-Freewater, Oregon, community for over 50 years.
He was born in Parkland, Washington, and died in Corvallis, Oregon, at age 93. He was the son of Bjug Harstad, founder of Pacific Lutheran University. He was buried at Oaklawn Cemetery in Corvallis.