The 25-year-old's sensational form on Wearside was this week rewarded with a call-up from national boss Craig Levein as the pair settled their differences.
As a result, he will head off after Saturday's Barclays Premier League clash at reigning champions Manchester City to join up with his international team-mates for the first time since February last year ahead of their crucial World Cup qualifiers against Wales and Belgium.
He will go with the blessing of O'Neill, who has already reaped a rich reward for his £12million summer investment after seeing the striker plunder five goals in four league games.
However, the Ulsterman will spend the 10 days or so he is away hoping that Fletcher returns in good health and ready to pick up where he left off for the Black Cats.
O'Neill said: "He hasn't spoken much about it to me. I think he's pleased that whatever impasse was there has been breached and that he is back in the fold again.
"From my own viewpoint, I am obviously delighted for him that he is back in.
"Of course, it's another one to add to the concerns about him coming back in a fortnight's time, but you just take that into consideration.
"I think he is pleased to be back and from Scotland's viewpoint, it's good news for them.
"For him, it's good he's back there. I think he will improve the team. He is playing splendidly for us and I think he will be a boost for Scotland."
Fletcher will be one of a dozen men, including England Under-21 internationals Danny Rose and Connor Wickham, who will join up with their respective national squads after the City game.
O'Neill, who insists he was not the third party who brokered the peace deal between Levein and the frontman, admits it will be an anxious time for him.
He said: "It's obviously a big concern for myself, a big concern for other managers in my position in the Premier League who have got players away.
"As [Newcastle boss] Alan Pardew said a couple of weeks ago, you are never sure what sort of training they are doing and how they are reacting to it.
"There's a pattern at club level, then you go into international games and international managers have a different way, of course - everyone does have - and it can upset their rhythm a little bit.
"But those are things you have just got to get on with. Thankfully from a club viewpoint, this is the last international break - big break, anyway - for quite some considerable time, so I will feel the season has really started then, I suppose, just after that."
O'Neill's concern is perhaps understandable - Adam Johnson, his other big-money signing, returned from the last England camp with a thigh injury which has kept him out of much of his club's programme since, and he will join up once again.
The Sunderland manager said: "It's interesting, he did the injury on international duty and so we have missed his services for most of that time since the last international game.
"But that's the way it goes."
Meanwhile, O'Neill was hoping for a boost this afternoon when long-term absentee Phil Bardsley stepped up his return from injury in a behind-closed-doors friendly against Middlesbrough.
Out-of-contract former Everton striker James McFadden, who has been training with the club, was also due to play with talks over his future imminent.
O'Neill said: "I am going to speak to his agent and I am hoping we will be able to come up with something. It's encouraging."