What makes you think a democracy can't be oppressive?
Even in perfect democracies there is something called the tyranny of the majority, where the majority can oppress the minority.
If we're talking about the US in particular, we have to recognize first that it's not even a perfect democracy, and there are many anti-democratic things about it such as the electoral college, and plenty more things that hinder democracy even where it exists (such as poor civic education, money's outsize influence in elections, extremely biased media, branches of government which shirk their balancing and oversight roles, etc).
Then, to get specifically to the oppressive aspects of the US, they range from slavery and lack of women's rights from its foundation, to segregation that existed in law up to the middle of the 20th Century (and arguably still exists in fact to some extent and in some places in the US even now), to the imprisonment in concentration camps of Americans of Japanese descent, to discrimination against people who weren't heterosexual, to the War on Drugs and police brutality which primarily impact minorities, to abuse, killing, and imprisonment of people who come to the US from other countries.
All this oppression and more has happened in what is ostensibly a democracy, and often likes to style itself as the world's greatest democracy.
And all of this oppression has had to do with identity, which required identifying people's race, gender, sexual preferences, or country of origin.
Such identification is amplified and made all that much easier in the age of computers, the internet, and gigantic databases on everyone. It's a data trove just begging for abuse.