Oppressive is that which seeks removes your positive freedoms. Positive freedoms should be fairly uncontroversial, things like freedom of opinion, freedom to bodily autonomy, freedom to seek work, freedom to basic living conditions etc.
To ensure they don't become politicized, any new positive freedom should require a 2/3rds approval by the government (and for the people in the US that means you need to figure out multi-party or this is gonna suck) and removing a positive freedom would require 3/4th approval by the government.
The procedures and laws should be handled and drafted by a separated government branch, which is dedicated to protecting the freedoms granted by the constitution (as well as protecting the constitution of your country itself). They substitute prosecution in case of violations but do have to go through the same courts as everyone else.
To ensure that this doesn't escalate for person-on-person disputes, generally they should only prosecute those who are threatening to oppress or have oppressed a group of people or a person for being part of a group of people under protection from the constitution (like their sex, gender, economic background or skin color).
