A short story does not necessarily have to have a message. As a work of art, a short story is intended to produce "a single effect." That is part of Edgar Allan Poe's definition and is now widely accepted. In order to analyze a short story it is a good idea to start with the single effect it produces. Poe used the term "effect," but we might call it the "feeling" or "emotional effect" produced by the story and left in the reader's mind after it ends. In Shirley Jackson's famous story "The Lottery," for example, we would probably all agree that the "effect" or feeling is one of horror and disbelief as Tessie Hutchinson is encircled by her own relatives, friends, and neighbors, all of whom are intent on stoning her to death. In "An Occurrence at Owl Creek Bridge" the effect is one of pity and compassion for the poor man who thought he had escaped from the enemy soldiers and had made it all the way back to his plantation and to the loving arms of his wife.
As he is about to clasp her he feels a stunning blow upon the back of the neck; a blinding white light blazes all about him with a sound like the shock of a cannon--then all is darkness and silence!
Peyton Farquhar was dead; his body, with a broken neck, swung gently from side to side beneath the timbers of the Owl Creek bridge.
What was the author's purpose in producing that "effect" in his reader? We wish it hadn't happened. We wish Peyton Farquhar had been able to reach safety, even though our sympathies may be with the Union cause. We feel the terrible senselessness and inhumanity and irrationality of war. It is a kind of mass insanity that overwhelms people periodically. The "occurrence" at Owl Creek Bridge was just one little incident in a great conflict that resulted in at least 750,000 deaths. Farquhar was only one individual of the many who just wanted to live and had their lives cut short. Will this always be the case? Or will men someday find a way to abolish war?
I have always viewed this story as the perfect example of the old saying about how a person's past life flashes before their eyes in the final moment of death. Such was the case with Peyton Farquhar.
I tend to look at the message of the text as being one which highlights the way one's life allows them to pass peacefully onto the next (if that is what one believes). Farquhar uses his memories to make his death easier.
I would say one of the themes is fantasy versus reality. Although we would like to think Peyton Farquhar will escape, and he pretends he does, the reality is that he is actually hanged.