Oryx and Crake is about a time in the future. It centers in on Snowman, whose real name is Jimmy. This Snowman character finds himself in a world where only he and a handful of other "people" live in. These people were created by Crake, the mastermind behind all that's happened, including behind the end of humankind with the exception of Snowman and the "people".
The character Oryx is a woman who Snowman, or Jimmy, had a relationship with. Throughout the book you learn how she ends up dying. She and Snowman loved each other very much. Still, she had some confusing feelings towards Crake.
Crake doesn't get to live either. He dies along with Oryx.
Snowman must learn to live with these "people" that were created to be perfect and flawless. They are harmless and innocent, victims of Crake's notorius mind.
Snowman believes no one is out there, no true human is out there but himself. He sometimes tries to kid himself that there is someone out there like him but he know for sure it is just not possible.
Yet, at the very end of the book, he is told by the "people" that they saw others that looked like him, like Snowman. Snowman decides to go and look for the others. He finally reaches them and observes them. He can see that they are carrying weapons for protection and that they look fairly normal, like him.
He now must make a vital choice: go to them and let them see he's alive or stay behind and hide himself as well as hide the "people" he needs to care for.
(Snowman is Jimmy's nickname. He gave it to himself when the end of the world came about).