Aaron Hughes has left the Northern Ireland squad and returned to Hearts due to injury.
The 38-year-old has made four appearances for Hearts this term but hasn't featured since August 18, struggling again with an injury that had caused problems during last season.
"He just has a little bit of a recurrence of the calf (problem), " said O'Neill. "We knew it was there when he came in and unfortunately he's just not really up to speed enough to be considered to play so he has returned to Hearts."
That means O'Neill has a 25 man panel to choose from in tomorrow's UEFA Nations League opener against Bosnia & Herzegovinia and Tuesday's friendly against Israel. Both games are taking place at Windsor Park.
Goalkeepers: Michael McGovern (Norwich City), Trevor Carson (Motherwell), Bailey Peacock-Farrell (Leeds United).
Defenders: Craig Cathcart (Watford), Jonny Evans (West Brom), Gareth McAuley (unattached), Conor McLaughlin (Millwall), Paddy McNair (Middlesborough), Michael Smith (Hearts), Jamal Lewis (Norwich City)
Midfielders: Steven Davis (Southampton), Niall McGinn (Aberdeen), Stuart Dallas (Leeds), Corry Evans (Blackburn), Shane Ferguson (Millwall), George Saville (Millwall), Jordan Jones (Kilmarnock), Oliver Norwood (Sheffield United), Gavin Whyte (Oxford United)
Forwards: Kyle Lafferty (Rangers), Liam Boyce (Burton Albion), Josh Magennis (Bolton Wanderers), Will Grigg (Wigan Athletic), Conor Washington (QPR), Jamie Ward (Nottingham Forest).