R.D. Dawe, ed., Sophocles, Oedipus Rex
(Cambridge: Cambridge University Press, 1982), p. 142 (on line 486):
There is an interesting essay on ὀπίσω 'afterwards' by Jonas Palm, 'Lag die Zukunft der Griechen hinter ihnen?' ('Did the future of the Greeks lie behind them?') in Annales Academiae Regiae Scientiarum Upsaliensis 13 (1969) [5-13].
I don't have access to Palm's article. See Liddell-Scott-Jones, s.v. ὀπίσω
, sense II:
of Time, hereafter, since the future is unseen and was therefore regarded as behind us, whereas the past is known and therefore before our eyes...