The former Wolverhampton Wanderers forward held talks with Scotland boss Craig Levein over returning to the team and was subsequently included in the squad to face Wales and Belgium this month.
"He hasn't spoken much about it to me," Sky Sports News quotes O'Neill as saying. "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."
Fletcher has netted five goals in four appearances for the Black Cats.