Chorney, 24, had a goal and three assists in 12 games for the Oilers last season.
The six-foot, 193-pound defenceman has one goal and six assists in 56 career games since being drafted by Edmonton 36th overall in the 2005 draft.
O'Marra, also 24, had one goal and four assists in 21 games for Edmonton last season.
The 6-2, 220-pound centre has one goal and five assists in 24 games since the New York Islanders drafted him 15th overall in 2005.
Note to readers: This is a corrected story. A previous version had O'Marra drafted by Edmonton instead of the New York Islanders.