The OA class is a solitary steam locomotive built by the Baldwin Locomotive Works for the Wellington and Manawatu Railway (WMR) in New Zealand.

In 1908, the WMR and its locomotive fleet were purchased by New Zealand Railways Department (NZR) and incorporated into the national rail network, and, although No.

13 bore a likeness to members of the O class, it was sufficiently different that it warranted separate classification.

[1] In 1896, a locomotive similar in appearance was ordered by the WMR, No.

Its technical specifications were such that when it was acquired by NZR it was classified separately and became the sole member of the OC class.