All taxes are inherently oppressive. As I quoted earlier, 'the power to tax is the power to destroy.'
We agree to a certain amount of oppression when we implicitly agree to abide by the social contract. We expect certain benefits to being in an organized society, such as the free flow of commerce - which requires roads; protection - which involves the military, police, fire, EMS, jails, etc.; basic education for children - which requires schools; and so on. That takes money, and we agree to give up a certain amount of our money to fund those benefits.
The question isn't when taxation becomes oppressive, but how much oppression we're willing to accept.