Both players were free agents.
"These players provide us additional organizational depth," said Avalanche assistant GM Greg Sherman. "We have the ability now with our affiliates to sign and develop players with our own staff and philosophy."
O'Neill, 21, played four years at Notre Dame University. He finished second among Irish defensemen in points last season with 21. The native of Essex, Ont., was originally drafted by the New York Islanders in 2004.
The 30-year-old Smith has appeared in 186 NHL games with Minnesota, Nashville, Phoenix and the Islanders. He played a career-high 61 games with the Wild last season and registered six points (3-3).