Fletcher has been called up for Scotland's World Cup qualifier against Wales and Belgium, ending a 20 month stand-off between himself and Scotland boss Craig Levein.
Sunderland and Scotland defender Phil Bardsley acted as intermediary between the two men as Fletcher and Levein put their differences aside, as both parties agreed that "the country comes first".
Fletcher is delighted to have been recalled to the Scotland side and moved to thank O'Neill and Bardsley for their support.
In an interview on scottishfa.co.uk, Fletcher said: "I am just happy to be back as part of the squad and looking to make a positive contribution in what are two massive World Cup qualifiers.
"I am a more experienced player than I was two years ago and I am delighted with the way things have gone at Sunderland.
"The manager [Martin O'Neill] has been very supportive, as has Phil Bardsley - who is gutted to be missing through injury."