How do ideas about "normal" and "abnormal" human behavior influence the experience of the individual? Taking this a step further, how does the concept of normal vs. abnormal become a form of oppression?

In the earliest stage of development, humans learn which rules of social behavior are accepted in varying social situations. Through experience, (most of the time) people conform to the unwritten rules of social context, which varies from culture to culture. Part of this is a human desire to be able to predict how any group of people or even an individual will react to any given stimuli. Thus, there are always social expectations about how individuals will act and react, and thus we internalize a code of "normal" and "abnormal" behaviors.
Another way to think about an "abnormal" behavior is that it is statistically less likely to happen. Some traits that are atypical don't affect human behaviors or experiences at all. For example, if someone is left-handed, that likely doesn't affect anything about their perception of themselves; likewise, it likely doesn't affect the way anyone treats them. However, it is worth noting that a lot of objects are designed for right-handed people, and people who are not right-handed must buy specially designed items to accommodate their left-handedness. In this way, left-handed people are consistently confronted with their "abnormality."
Which atypical patterns of behavior then create some form of altered experience for an individual? Much of this depends on the societal reaction to the unexpected behaviors. The feedback provided to the individual creates a feedback system that is either supportive to the individual or discouraging to them. It is necessary to consider several things when judging the impact any atypical behaviors will have on an individual:
the degree to which a person's behavior falls outside the normal expected range of behaviors
how important that societal norm is within the culture
any values a culture attaches to the variance from typical behaviors
There are so many ways that any individual's behaviors can fall outside the ranges of these parameters that likely everyone is deemed atypical in some category at some point over the course of a normal life span. However, some people fall so far outside the realms of normal and expected behaviors that they cannot function in a society that is constructed on the basis of these norms.
Rosenhan & Seligman (1989) suggest the following characteristics that define failure to function adequately:

SufferingMaladaptiveness (danger to self)Vividness & unconventionality (stands out)Unpredictably & loss of controlIrrationality/incomprehensibilityCauses observer discomfortViolates moral/social standards

Receiving a diagnosis or classification existing outside the realms of normalcy can cause stress for individuals and their families. While most people lay claim to wanting to live life as an individual, they typically want to do so within the confines of socially accepted, typical classifications. To be set apart in ways beyond one's control can feel isolating and dehumanizing. Again, the reaction from family and friends is crucial in determining how well an individual is able to cope with any atypical classification.
Oppression occurs when people who have the power in a society define what is "abnormal" as anything that is inconsistent with their own behavior (the "norm"). In this context, then, people can be oppressed when those who fall within "abnormal" realms are denied fundamental human rights or privileges that extended to the majority of members in a society. Examples of this could look like the following:
Students with learning challenges do not excel in school, since the educational system is designed for people who don't have these challenges
People with atypical behaviors are seen as invisible in public spaces (e.g., homelessness, as an "abnormality," is ignored)
Applying for a job as a physically differently-abled person is extremely difficult due to discrimination from employers or even from physical access these spaces (statistics show that such people are more likely to be turned down for jobs)
It is important to advocate for those who are "abnormal" or atypical in our society. It is important to call out oppression when you see it and to advocate for people and policies who seek to diminish these discrepancies. It also requires simply educating people about how societal norms affect those who fall outside the realms of normalcy—and how to better support people who need it.
https://www.simplypsychology.org/abnormal-psychology.html