In the movie, Maddy's mother says Olly needs a haircut. In the book, Olly has a shaved head.
Olly's life at home was a bigger role in the book than in the movie. The book goes more into depth on Olly's life and his person.
There is a scene in the movie where the astronaut is sitting on a bar stool in a diner that Maddy created. This scene is a reference to the famous 1942 painting "Nighthawks" by American artist Edward Hopper.
When Olly and Maddy first exchange phone numbers, we can see that the last four digits of his number read 0117, which look like a numerical spelling of "Olly."
In the movie, Olly has brown colored eyes. In the book, Olly is written to have ocean blue colored eyes and an olive complexion.