It will be her only public appearance to talk about the book.
She'll also be interviewed on stage by actor and comedian Stephen Fry, who has narrated Harry's adventures on the radio.
OOTP, which comes out on 21 June, is already breaking records as the most pre-ordered book in history.
It's the fifth book in JK's Harry Potter series and fans have been waiting three years for it to hit the shops.
The first four novels have now sold almost 200 million copies in 55 languages and 200 countries.