The headword is an adverb, on which see generally LSJ s.v. It occurs with the epic (double pi) spelling, o(ppote/rwqen, at Homer, Iliad 14.59 (web address 1: Nestor observes that the battle is going so badly for the Achaeans that one cannot discern from which of the two sides they are being driven in rout). The later and more normal spelling, as here, occurs in e.g. Aristotle, Parts of Animals 691b10, discussing from which of the two directions (i.e. top or bottom) the biting power of a river crocodile is greater. See also e.g. Josephus, Jewish War 6.75.
Adler also cites Lexicon Ambrosianum 418.
[1] The headword is identically glossed in the Synagoge, Photius' Lexicon (omicron417 Theodoridis, where the editor supports Coraes' proposed emendation of the first two words of the glossing phrase from a)po\ poi/ou to a)f' o(poi/ou), and Lexica Segueriana 319.18. (Adler also cites Lexicon Ambrosianum 416.) Cunliffe, s.v. o(ppote/rwqen suggests from which quarter (p. 297).