Oppression is categorism in a severely unequal balance of power. A balance where a person, group or social structure doing categorism has a strong power advantage over the human beings who this categorism is being done to. (In Swedish: förtryck)
Categorism exists on many different levels: Individual, group, systemic, structural, discursive and dogmatic. Would this mean that oppression is categorism on the structural level? Not always. While much oppression is structural, a dictionary definition1 of oppression is “unjust or cruel exercise of authority or power”. This is something that can be done on any level. Structural, local group or individual. While a definition limited to a structural level may be useful in some contexts, the word itself is more flexible with other relevant meanings that should not be ignored. Ultimately, oppression is about power, and power is something that exist between people. Exist on all levels of social interaction. The discourse about racism, homophobia or any other focus of categorism should not be restricted to the individual level, to the socioeconomic and/or cultural structural level, or any other one level. Instead, it need to be able to handle all of the levels. Separately, as well as in interaction with each other.
All oppression is categorism, but not all categorism is fueled by enough power to be oppression.
For more on the concept of oppression, see chapter "9.2 Conceptualizing oppression" of the thesis.