CN O-18-a is a class of 0-6-0 steam locomotives formerly owned by Canadian National Railways.

They were switchers built by the Grand Trunk Railway's Point St. Charles Shops.

The Grand Trunk Railway built 50 of these locomotives between 1919 and 1921 and designated them F9 Class.

CN classified the homebuilt locomotives O-18-a and the Lima-built F9s became GTW O-18-b class.

The O-18-a locomotives were numbered 1749–1798 by the Grand Trunk, and were renumbered as 7423–7473 by the Canadian National.