What is an oxymoron?
An oxymoron is a seemingly contradictory statement.
On the surface an oxymoron seems to be contradictory, for example, "Child is father of man". On first inspection how can a child be a father of man? Of course a child cannot but a child grows into an adult and may well become a father of a son.
Another example is "The King is dead, long live the King". Again this seems to be contradictory but this assumes we are talking about then same person.
However we are talking about two different people. When George 1 died, George 2nd became King so in this context the statement makes sense.