Solution:

First we put 3 oranges on the left and 3 on the right-hand side of the balance. In the case that the balance shows equal weights, one of the remaining two oranges is lighter and in the second measurement we can easily see which one. However, if one side (say left) is lighter, then we know that the orange we are looking for is one of the three that appear on the left side. Let us denote these oranges by $A$, $B$, $C$. In the second measurement we can measure $A$ against $B$. If one of them is lighter than the other, we are done (as the lighter orange is the one we are looking for). If the weights of $A$ and $B$ are the same, then $C$ is the lighter orange.