Green’s Dictionary of Slang

(orig. US) a Western film; also attrib.

1948		W. Tucker Dove 35: ‘What is a hay-burner?’ Elizabeth interrupted. ‘A horse opera,’ Horne provided. ‘An oater, a Western.’ [DA].
1951		Green & Laurie Show Biz from Vaude to Video 570: Oater – Western film.
1962		S. Longstreet Flesh Peddlers (1964) 176: The merits of ‘soaps’ and ‘oaters’ [...] western action stories.
1979		in D.H. Kirkley Network Television Western 95: [It] proves to be little more than a patchwork of stock oater ideas without a fresh twist to distinguish it.
1990		Leonard Maltin’s TV Movies and Video Guide 1260: Flabby oater about capture of outlaws who killed Indian squaws.
2000		J.P. Langellier Custer: The Man, the Myth, the Movies 1: The lowbudget oater was destined to disappear without a trace.
2021		J. Ellroy Widespread Panic 96: Jimmy’s [i.e. Dean] inked for some big oater set in Texas.