Oater

~1945-50 - From oat as in horse fodder, alluding to the horses common to the movies, + -er "connected to; about".

Full definition of oater

(plural oaters)

(informal, humorous) A movie or television show about cowboy or frontier life; a western movie.

1949 January 10, The Great American Horse Opera, in life (magazine),In recent years the western or horse opera, known in the trade as the "oater," has come to be recognized as an art form just as formal as the ballet or the symphony. In essence it is the American morality play. To prove his contention that all this is so, Life Photographer John Florea took these unusual pictures during the filming of Yellow Sky. This is a $1,450,000 western with big-name stars (Gregory Peck, Anne Bancroft, Richard Widmark) and technical talent from 20th Century's top drawer, but is basically a typical oater.
1995, Louis Decimus Rubin, Jerry Leath Mills, A Writer's Companion,By far the more common was the low-budget "hoss opera" or "oater," ground out in relentless numbers by studios such as Universal and Republic, and designed basically for edification of the young, who took them in on Fridays and Saturdays along with the episode of a serial, a cartoon, a newsreel, and perhaps a bouncing-ball sing-along. There were, to be sure, degrees of the oater; a somewhat more subtle version, designed for adult as well as child viewing, was also made.