O'Neill not sure he has 'energy' for long Celtic stay
The 73-year-old is back at the Glasgow club for a third time following the departure of Frenchman Wilfried Nancy, who was dismissed following six defeats in his eight games in charge.
O'Neill took charge on an interim basis after Brendan Rodgers' exit in October, winning seven out of eight matches.
Nancy took charge early last month but his reign quickly turned sour.
Former Aston Villa and Ireland boss O'Neill will again be at the helm until the end of the season but he is unsure about a longer-term stay.
"I can't see that for a start and I genuinely can't. I don't know whether I would have the energy and all that type of stuff," O'Neill said on Talksport when asked if he would feel like staying on should the campaign finish on a positive note.
"I will just put everything into it now for these next couple of months and the same applies to me -- if I don't win football matches I'll be straight under pressure and that starts here at the weekend."
When O'Neill finished his interim spell, Celtic were level on points with Scottish Premiership leaders Hearts with a game in hand and nine points ahead of Rangers.
But they are now six points behind Hearts with the same number of matches played and would be three off Rangers in third if the Light Blues beat Aberdeen at Ibrox on Tuesday.
"It will be very, very difficult -- we are behind," O'Neill said on Celtic's title hopes.
"Rangers have caught up now as well too and so it will be difficult, but it's a challenge for us."
O'Neill previously managed Celtic on a full-time basis from 2000 to 2005, winning three Scottish Premiership titles.