Veteran goalkeeper Shay Given announced his retirement after Ireland's Euro 2012 campaign, but has since made himself available once again and has been included in O'Neill's 27-man party for the clash with Oman and the Euro 2016 qualifier in Georgia on Sunday.
The 38-year-old is one of four keepers in the squad, alongside David Forde, Keiren Westwood and Rob Elliot.
That leaves O'Neill with a decision to make over who gets the nod, but O'Neill insists that Given, who has 125 caps to his name, is not guaranteed a starting berth.
"I talked to Shay quite some time ago way back last season when he was on loan at Middlesbrough and he had expressed an interest in returning at that particular time," he said.
"He was playing a few games then. I was new to [the job] myself, just having a look at the situation, and he still has that desire to come and play.
"But no promises have been made whatsoever. David [Forde] has done well for me. Why shouldn't there be competition for places? I haven't picked the side yet.
"Players who are in the team and have done well shouldn't have to worry too much but every single time you play, I think you have to prove yourself every single game."
After beating Latvia 3-0 in O'Neill's first match in charge, Ireland have failed to win any of their six matches since then and may be without record goalscorer Robbie Keane, who is "carrying a slight injury and his progress is being monitored."
For the visitors, experienced striker Imad Al-Hosni has been left out by coach Paul Le Guen after it was decided by the Oman Football Association that it was in the best interests of the player to remain at home and search for a club.
"Imad recently returned to Oman after completing his season with Al Nasr Club in the Saudi Arabian professional league," a statement read.
"He is without a club since July and the national team's technical staff, in consultation with Imad, decided that the priority for the striker was to find a club rather than travel."
