You can override or reject a decision if you're more powerful than the person who originally made the decision. And Congress has the power to override or nullify the Presidential veto if they have a two-thirds vote.
The word override can be used in a number of contexts. You can override or ride on top of the grass. You can override a horse, or ride it too hard. An army can override or dominate the enemy in a war. And a judge can override or set aside a decision in court. Regardless of which meaning you're using, remember that the thing doing the overriding always has the upper hand.