She, herself sees the book as fiction, based on autobiography, which seems to be backed up by the fact that it won the Whitbread Award for a First Novel in 1985.
From Jeanette Winterson's own website...
'Is it autobiographical?'
'Yes and no. All writers draw on their experience but experience isn't what makes a good book. As the stand-up comics say, 'It's the way you tell 'em'. Oranges is written in the first person, it's direct and uninhibited, but it isn't autobiography in the real sense. I have noticed that when women writers put themselves into their fiction, it's called autobiography. When men do it, such as Paul Auster or Milan Kundera it's called meta -fiction.'
Oranges are not the Only Fruit, Whitbread Award for best first fiction for the semi-autobiographical, religious excess and human obsession