Green’s Dictionary of Slang

1. an oil well.

1890		Columbus Dispatch 24 May n.p.: The Ohio Oil Co. [...] drilled in an oiler [...] that will be the largest one in this field [DA].
1950		Daily Ardmoreite 17 Feb. 1/6: [heading] Lone Grove Oiler Shows Good Signs [DA].

2. (orig. US) an oilskin or oilcloth coat and/or trousers.

1999

Observer Rev. 1 Aug. 2: They had to wear ‘oilers, the lot’ as the rain lashed.

3. (US) a Mexican [var. on greaser n.¹ (1a)].

1907		S.E. White Arizona Nights 69: There were a lot of renegades down toward the Mexican line [...] and a few oilers livin’ near had water-holes in the foot-hills.
1914		A.P. Man Jr ‘Further Word-List – Arizona’ in DN IV:ii 165: oiler, n. = greaser, Mexican.
1936		R.F. Adams Cowboy Lingo 198: To the cowboy a Mexican was a ‘greaser,’ ‘oiler,’ [...] or ‘bean-eater’.
1983		Maledicta VII 29: Mexicans were also called grease gut, grease boy, and oiler.