McClean jeered by Sunderland fans during victory over Fulham
The 23-year-old came in for criticism after opting not to sport a Remembrance Day poppy on his shirt during the Black Cats' 2-1 defeat at Everton last weekend.
"James will deal with it (the booing). I think it is to do with the issue last week, it's a free choice in this world," O'Neill said.
"James has lived with a lot of things - he's getting death threats too now so that doesn't help."