The new members will be honored along with anyone chosen in January in voting by the Baseball Writers' Association of America.
Hall of Famer and panel member Phil Niekro credited two historians on the panel, Peter Morris and Tom Simon, with helping to illuminate the accomplishments of those who are long gone.
"It's tough to go back into the 1800s and bring that to life," Niekro said. "It was so different then — five strikes, eight balls, batters can tell the pitcher where they want it. Can you imagine? I couldn't have done that if I tried, not with my knuckleball."
Ruppert, O'Day and White all died in the 1930s — the first Hall class was selected in 1936.
Hall of Famers Bert Blyleven, Don Sutton, Pat Gillick and Niekro were among the voters who considered 10 candidates. Former NL MVPs Marty Marion and Bucky Walters also were on the ballot.
It took 75 percent (12 votes) for election. Ruppert and O'Day each got 15 votes and White drew 12. Bill Dahlen got 10 and Marion, Walters, Sam Breadon, Wes Ferrell, Tony Mullane and Alfred Reach each got three votes or less.