The 21-year-old keeper has rejected approaches by England to switch his international allegiance, but if he played in a competitive fixture - such as Northern Ireland's opening match in the UEFA Nations league - his fate would be decided once and for all.
Peacock-Farrell was born in Darlington, but qualifies to play for Northern Ireland through his grandfather.
The situation puts pressure on O'Neill to cap the youngster as soon as possible, or face losing him. However, the manager doesn't seem to be feeling it.
"At the end of the day I don’t think we’ll lose Bailey. He’s committed to us. But it’s a competitive game and the start of a new tournament. The goalkeeper that is chosen tomorrow is the goalkeeper I think deserves to play," said O'Neill.
"This is a competitive game with a route that we've talked about to Euro 2020, so the goalkeeper that is picked on Saturday will be the goalkeeper that deserves to play."
Peacock-Farrell faces competition from Norwich's Michael McGovern and Motherwell's Trevor Carson.
O'Neill was full of praise for the young player, saying "We found Bailey really when he was potential, nothing more than that. He’s done an awful lot off his own back to get where he is and be first choice at Leeds."