Around half a million copies of OOTP are expected to be delivered by the post office on Saturday 21 June.
The extra vans and staff should make sure that none of the young (and old!) fans are disappointed, a Royal Mail spokesman said.
But they did warn people to stay at home if they're expecting the book to come through the post.
Because the book is so large - it's 768 pages long - it won't fit through most people's letter boxes.