Back in Selma today for the 50th Anniversary of "Bloody Sunday," Olsen, a retired white minister, now 81, told ABC News about the night of March 9, 1965. As Olsen and Reeb, who was also a white minister, were walking to where Martin Luther King was going to speak, four white segregationists attacked Reeb on the street.
"One of them swung a club and hit Jim on the head. Right above his left ear," Olsen said. "Smashed his skull."
Reeb "was incoherent, babbling," Olsen said. Reeb was hospitalized and died 37 hours later.
"It was a big press event," Olsen said. "Martin Luther King announced it."
President Johnson even sent yellow roses to the hospital, Olsen said.
"It's become a very important day in my life," Olsen said. "It's been an enormous gift to me to be able to speak about it.
"Also to know that the death of this white minister was really ... the tipping point for the passage of the Voting Rights Bill," he added.
Johnson, who urged the passing of the bill the following week, referred specifically to Reeb's death, Olsen said.
Olsen said he is "very much aware" that he could have died instead of Reeb.
"It's a gift to have lived," Olsen said."
But there is "work that needs to be done," Olsen added.
"It's 50 years since the Voting Rights Bill was passed and we still have not fully created equality in voting," he said. "We still have a lot of inequality in our society. My message ... is that whenever you see something wrong, step up, say something. Tell somebody. Take a stand."