The 19-year-old Aston Villa midfielder was criticised by both Ireland and England fans on Twitter for delaying a decision on which of the two countries to represent at senior international level, although there was also support from both sides for the emerging talent.
An ongoing debate was rekindled on Tuesday when the teenager's name did not appear in a 33-man provisional squad for the Republic's friendly against Roy Hodgson's men and the Euro 2016 qualifier showdown with Scotland next month, with O'Neill revealing he had asked the player if he would reconsider his decision to leave it until the summer to jump one way or the other.
The Republic boss made his move after seeing Grealish win rave reviews for his performances at club level in recent weeks, persuading him that the time was right less than two months after admitting he was happy to let the situation ride.
Asked what had prompted him to change his mind, O'Neill said: "Well, of course he has played a lot of games since then.
"He had not been a regular member of the team and when he did play, it was generally as sub
Under Tim Sherwood, he has started quite a number of games - not that I would start pre-empting Tim's teams - but he has made a very good impression from the start.
"Obviously as these games go along, he'll get fitter too by starting matches - he seems to be lasting longer in the games too and doing a lot of good work - so of course, the last four or five weeks have made a big difference to Jack's career and obviously to my thinking.
"He is seen now as not just part of the squad, but seemingly part of the team, and people's aspirations change a little bit during the course of weeks, never mind months, so it might be something he might look at differently.
"But he did say he didn't want to make a decision now."
Birmingham-born Grealish has been courted by both the Republic and England, but has already represented Ireland at under-age levels.
He indicated earlier this year that he might take the Under-21s route with the Irish, but asked if he was confident the player would declare in his favour, O'Neill said: "Do you know what? I really don't know at this stage, but I'm sure there's still a good possibility."
Source : PA
Source: PA