The book company said the publication date was brought forward because loads of illegal copies of the book had turned up.
It's even offering a reward to anyone who helps it catch the people behind the scam, who are known as 'pirates'.
OOTP came out in the UK on June 21.
Just a month later, Chinese translations started appearing online.
Soon after, illegal versions of OOTP turned up in Chinese bookshops.
The previous four Potter books proved poular with Chinese readers - with a total 5.6 million copies printed.
Wonder if they'll like OOTP too?