obie, n. — Green’s Dictionary of Slang

obie n.

also O.B., obey

[ety. unknown; Irwin (1931) suggests: ‘Originated by the old yeggmen, who merely reversed the initials, “P.O.,” to lessen the public’s understanding of their conversation, and from carelessness in speech corrupted to its present form’]

(US Und.) a post office.

1927		C. Samolar ‘Argot of the Vagabond’ in AS II:9 385: An obie, or O.B., was a post-office.
1928		J. Callahan Man’s Grim Justice 46: ‘There’s nobody sleeping over that obey (post office)’ he went on.
1933		Ersine Und. and Prison Sl. 55: obey, n. A small-town post office.
1962		Ragen & Finston World’s Toughest Prison 810: obie – A country post-office.