Oater

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. […] 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 […] and technical talent from 20th Century's top drawer, but is basically a typical oater.

Source: wiktionary

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[…] 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.

Source: wiktionary

While trying to run a trading post at the depths of the Great Depression, Harry Goulding heard that a Hollywood director was looking for the perfect Western backdrop for his next oater.

Source: wiktionary

Akira Kurosawa’s samurai movies were already heavily influenced by the western genre, so it wasn’t hard for Hollywood to convert Seven Samurai (1954) back into a bona fide oater.

Source: wiktionary