Cameron Carter-Vickers is back in action after an over eight-month injury layoff.
The American defender, who suffered an Achilles injury against Sturm Graz last October, played a behind-closed-doors game at Lennoxtown and 45 minutes in Dublin this week.
Carter-Vickers said: "I've been training for a little while.
"I trained back into the last season and then the first couple of weeks of pre-season, so I was pretty confident I would be fine injury-wise, but yes, it's good to get back playing football and build up that football fitness."
The central defender had been absent for most of Celtic's last season, watching from the sidelines as the team secured the double.
Recalling the experience, he said: "It was crazy. If you know you're going to win it like that, that's probably the most exciting way to do it.
"I would have liked it to have been a bit more comfortable.
"I think any player would definitely rather be playing than watching.
"You definitely feel a bit more nervous when you're not out there."
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Carter-Vickers has yet to play a significant game under O'Neill, but he witnessed the manager's influence on the team last season.
He was quick to praise the role of captain Callum McGregor in Celtic's triumph, saying: "When I look back at last season, I do think Callum McGregor, the captain, was very important in that as well.
"Just the way he leads us day in, day out. He brings energy and pushes the team forward."
Celtic are looking to bolster their squad for the upcoming season.
They have already extended Alex Oxlade-Chamberlain's contract by another year and are in talks to re-sign Marcelo Saracchi and Kelechi Iheanacho.
O'Neill is also keen to sign a new goalkeeper, and Colombian attacker Camilo Duran is set to join the club for a fee believed to be around £5.5 million.
Carter-Vickers said: "I think it's definitely important that we strengthen the squad.
"Obviously, there are players out of our hands, but if you're telling me there's a striker coming in, then that's good news.
"Hopefully a few more."
Despite being back in pre-season with Celtic, Carter-Vickers could have been representing the United States in the expanded World Cup this summer.
However, his injury ruled him out of contention.
He said: "I got injured back in October. I kind of knew at that point it was going to be very hard to get back fit before the end of the season. So I kind of processed that a long time ago. I'm fine with that."
He made just two appearances in the competition, scoring in a 3-2 group stage loss to Turkiye.
The US were eliminated in the last 16 by Belgium after a 4-1 defeat in Seattle. It is the only one of their games Carter-Vickers missed.
"I watched a few of them. I didn't watch the last one (against Belgium), but I've kept up as much as I can," he added.
At 28 years of age, the 2030 World Cup in Spain, Portugal, and Morocco could still be in reach for Carter-Vickers.
Before then, he will focus on returning to full fitness with Celtic in what is his seventh full season on their books.
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