In this case, this is several unrelated words strung together to describe the characrter. Together, they don't mean anything different than each word separately.The writer means to describe the character as being a poor white man from the country (not a city), who seems out of place in his new environment and was admitted to the school because he was poor and unusual, or was not privileged like their usual students. Oddity refers to someone or something who is unusual or out of place. Affirmative Action representative refers to a US Legal called Affirmative Action, which made it a priority for universities to accept students from poor backgrounds, minority races, women and those from underserved areas/neighborhoods. If someone is an "affirmative action representative" he was admitted to the school because he was poor and underserved, not because he was the most qualified student who applied for the position.

1. Oddity - noun for "odd". This means he is an odd man, a weird person. 2. Affirmative action -  "Affirmative action in the United States tends to focus on issues such as education and employment, specifically granting special consideration to minorities" (quoted from Wikipedia), who have long suffered discrimination. The idea is to use affirmative action as a tool to reverse the discrimination. Here it is used as self-deprecating humour: he is saying that white and poor rural dwellers are a minority and that he himself is an odd man. He is saying that he would not normally qualify but was only admitted as a result of affirmative action. Normally black and poor rural dwellers are a minority that suffer discrimination. In some other countries affirmative action is called positive action.