Walker, 17, finished last season as Preston's youth team's top scorer for the second consecutive year.
He has also appeared for Altrincham on loan, making four appearances for the National League North side.
O'Reilly, 18, made his first-team debut in the same game as Walker against Aston Villa in 2018.
He has also featured for the Republic of Ireland's under 17s six times, scoring one goal.
They both join on loan until the end of the season.
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