How would I know what was going through Oswald's head?
Exactly.
But you do realize, don't you, that Oswald's using his scope for such a close-up, fast-moving target at "Z-124" would be counterproductive, and if he did try that stunt (using the scope for his first, missing-everything shot), it would probably explain why he missed everything with it.
Either that, or, as Charles Collins hypothesizes, the barrel of his short-rifle bumped the box on the windowsill and caused
him it to prematurely "go off."
D'oh.