A figure of speech, oxymoron fuses two contradictory or opposing ideas. Everyone has heard oxymoron's in his/her life, and while they apparently contradict themselves, there is a certain logic to them. For instance, the phrase "Alone together" seems contradictory, yet there are married couples that are often seen together at restaurants and such where there is no communication between them; they each stare off somewhere with private thoughts and do not converse, or they each study phones before them, scrolling through emails, etc.
In Ender's Game the protagonist himself seems an oxymoron as in contradiction to his society, he is allowed to live as a third child. Also, he is chosen to be the leader although he is small. He is ruthless, but compassionate at the same time. His brother Peter is a "charming sociopath." This description is clearly an oxymoron.
Another oxymoron is Dr. Device the molecular destruction device used in space battles; for, normally a doctor repairs and heals rather than destroys.
A figure of speech, oxymoron fuses two contradictory or opposing ideas.
Ender's game portrays the fighting of an actual war with-in the training simulation of a game. The subjects believe they are training for war when in reality they are fighting a war. The very paradox provided is the oxymoron, the game and the war are one in the same. 'Ender's game' IS Ender's war. It is this very aspect that crushes Ender when he realizes that the game he thinks he is playing, is real, and not a game at all.