"An Occurrence at Owl Creek Bridge" contains morals or themes about versions of reality, time perception and justice in wartime. By the end of the story, the reader learns that all of these things are relative to the person experiencing them.
The story proves that there is not only one version of the truth or reality. As the story progresses, Farquhar's version of reality and what is happening to him become less believable even though he seems positive that everything he feels is real. This causes the reader to question what is real or unreal in his own life.
Similarly, time is skewed and things don't always seem to move at the same pace. Some events are described with lightning quickness while others barely creep along. Just like the story, a person's perception of time depends on what he is feeling or experiencing at a given point in time.
Lastly, the short story highlights justice in wartime. The story is all about a civilian being hanged for a crime without a trial. This shows how people in authority tend to do what they are allowed to do rather than what they should do, ethically.