What is the theme of "An Occurrence at Owl Creek Bridge" by Ambrose Bierce?
I think there is a main theme about time and how the perception of its passage is fluid. I'm quite certain most people have experienced how time seems to "fly" at certain times and "crawl" at others. The same occurs in this story. In the moments between the officer stepping aside and Farquhar's death, time slows dramatically. Farquhar experiences hours and hours of time in the fractions of seconds that exist before his neck snaps.
I think another theme is the theme of imagination/ dreams/ illusion vs. reality. When readers first meet Farquhar, we are told that he longs to fight for the Southern cause. He has dreamed up a romanticized version of what war is.
Circumstances of an imperious nature, which it is unnecessary to relate here, had prevented him from taking service with that gallant army which had fought the disastrous campaigns ending with the fall of Corinth, and he chafed under the inglorious restraint, longing for the release of his energies, the larger life of the soldier, the opportunity for distinction.
Unfortunately for Farquhar, the reality of war isn't as glorious as he imagines it is. The reality of the situation is that war is dangerous and brutal. Farquhar longs for distinction, but the reality is that no southerner will know what he tried to do. There will be no glory in his success (because he failed), and no one will celebrate his failed attempt because no one knows he went to try and sabotage the bridge.
The reality vs. non-reality theme continues in other parts of the story, too. The Union scout (reality) presented himself as a southern soldier (illusion). Farquhar imagines the information obtained is genuine only to discover that in reality it was all a trap. Farquhar also imagines a miraculous escape only to wind up dead in real life from hanging.