The WAGR Oa class were a total of 10 steam locomotives operated by the Western Australian Government Railways (WAGR) from 1909 to 1962.

The Oa were almost identical to the O class with the exception of the running board and footplate which were higher. The Oa class were constructed by Midland Railway Workshops due to a large wheat harvest that required more engines, the last to be built entered service in 1911.

The last Oa in service was No. 161, built in 1910 and withdrawn and scrapped in 1962.