No, it is actually not directly addressed in dialogue. We're shown a rebellion exists and that the Empire does some pretty hardcore things to those they consider enemies, but there's nothing actually in the body of the movie itself that communicates, "The Empire did really bad thing 'A' before the opening crawl, so the Rebels rebelled thusly." Events seen during the movie can't be used as the excuse for the Rebellion, unless cause and effect are reversed. It is to be assumed, based on the adjectives used to describe the Empire in the opening crawl and the potential power of the Death Star, that the Empire = Bad and the Rebels = Good, therefore, WAR!
The "evil Empire" has "sinister agents" and their Death Star possesses "enough power to destroy an entire planet." Why the Empire is evil, what makes these agents sinister or whether the Death Star has actually been used to destroy a planet are questions without answers...until you see the events of the movie, by which point the Rebellion is already in full swing.
I do find it curious that the simple possession of a weapon with enormous destructive capabilities (a WMD, as it were) is implied to be another mark on the Empire's Evilness Checklist. If the Rebel's had a Death Star, would they be slightly less Good, or would the Death Star simply cease to be evil?
As to the comment that there was "corruption and evil up to the very top" only a very few characters were shown to actually be objectively "evil" - Tarkin, Vader, Palpatine. And Motti's pretty bloodthirsty. These are the very top, but their subordinates did not appear particularly evil. They're pretty much all military men prosecuting a war against an openly declared enemy or rooting out traitors -and not falsely accused traitors, actual traitors. Leia and her entourage were all traitors to the Empire. They are Good because the Empire is Bad. If the opening crawl established the reverse, they wouldn't be valiant Rebels fighting an oppressive Empire, they'd be traitorous Rebels fighting a noble Empire.
A single squad of stormtroopers killed the Jawas, yes, but secrets vital to national security in a time of war had passed (unknowningly) through their little anything goes merchant hands, presumably heading for at least one Rebel agent. Without knowing the details of the event, I would categorize this under overly zealous and morally ambiguous. Luke's aunt and uncle were killed by the same group of stormtroopers on the same mission following the same secrets.
We don't actually know what happened during either event, only the end results: people died. Perhaps the Jawas violently resisted interrogation by the stormtrooper patrol and started a fight they couldn't win, a blaster went off and everyone opened fire. Perhaps the troopers realized they had over-reacted and tried to cover up what they'd done by blaming the Sand People. But then, why cover up the Jawa massacre and not the Lars'? The Jawas were just shot up and then Gaffi sticks and Bantha tracks were strewn around the scene. The Lars were burned down to skeletons and their home burned down next to them, no Gaffi sticks and one could presume, no false Bantha tracks.
Did the troopers harden from one event to the next? Did they feel the Lars' deaths were justified? Were they actually responding to a domestic dispute that went horribly wrong? Would the rest of Tatooine take more notice and be more outraged by the deaths of Jawa traders than Human farmers? Why would the Empire care? I thought they were an oppressive Evil which required rebellion. Rule through fear as Tarkin describes doesn't work when people fear someone else for things you've done. Cover up the deaths of a handful of Jawas but openly claim credit for the destruction of an entire world? Or did they dump a bunch of Gaffi sticks into the newly created Alderaan asteroid field and scratch some Bantha tracks on the bigger pieces of debris?
Descrimination against aliens? That's the EU, not the movies. The Rebels had three women, two black men, a single Asian man and maybe a dozen or so aliens, almost entirely of one other species (Mon Calamari) serving during the movies and most of those exceptions to the white guys only club were in ROTJ. Are they not sex-/race-/speciest because of these token appointments? A New Hope didn't show a single Rebel alien.
Limited movement? The Tantive IV was in receipt of stolen Imperial national secrets and carrying Rebel spies and resisted lawful boarding. The Millenium Falcon was unauthorized to leave a planet under limited blockade while a search for stolen national security secrets was conducted (Tatooine) and was detained upon recognition when it entered a system determined to be a Rebel stronghold (Alderaan). Which non-Rebel was limited in movement during the OT? Han, the known criminal aiding the Rebels? Lando, the known criminal harboring Rebels? Ewoks? The Empire ignored them until they attacked the garrison.
Lack of freedom in general? Where? Who? What freedoms of which non-Rebels were suppressed? Declared enemy combatants can have rights afforded them if there's an equivalent to realworld conventions of war in the GFFA, but enemies don't have freedoms to oppress.