Coach John Stiegelmeier made the announcement late Monday. He did not comment on the reasons for O'Brien's decision but told the Argus Leader newspaper that O'Brien was emotional after a lopsided loss last weekend at Cal Poly and told the coach he was "going to walk away" from football.
Redshirt freshman Austin Sumner likely will start Saturday against Illinois State in the Missouri Valley Conference opener for the Jackrabbits (1-2).
O'Brien is from Winona, Minn. He played in 25 games with the Jackrabbits and started 20 of them. His 4,376 career passing yards rank ninth all-time at South Dakota State.