So far I didn't find an earlier one than Dan Ingalls' "The Smalltalk-76 Programming System" (https://dl.acm.org/doi/10.1145/512760.512762) published in January 1978, what surprised me.

Remarkably the term is also used in developmental psychology, especially for child development; I found publications about "object-oriented behaviour" from the sixties, e.g. http://www.jstor.org/stable/1126427 from 1966.

Does anyone have earlier CS literature references than 1978?