I do not think that there is any one person who is an antagonist throughout the whole book. An antagonist is a character who works against the main character and tends to thwart her. Opal is clearly the protagonist, so who fits the bill as her antagonist?
Sometimes it is her father, who will not give her the information she wants about her mother. I would say that he is a major antagonist in the book.
I guess you could say it's the Dewberry boys, but I think they're more of a minor annoyance than a real antagonist.
So I guess I'd say that if it's a person in the book, it's her father. Otherwise, her antagonist might really just be her circumstances since her main conflict is really against her loneliness.