Tom Joad, as an ex-prisoner, is by definition an outsider, and prejudice against his criminal record means that he always has to hide from the police in the novel.
The episode of the camp in Hooverville shows how the Californians were prejudiced against Okies and used various stratagems to prevent their settling and obtaining work. The end of the section on Hooverville shows how the townspeople's prejudice against the Okies develops, and is in part fuelled by their own economic worries.
The chapter on the history of California shows how white settlers were prejudiced against Mexicans, and felt free to steal their land. Another inter-chapter discusses Native Americans. In some ways there are thematic parallels between the dispossessions of Mexicans, Native Americans, and Okies, all due to class and ethnic prejudice, and all aligned by Steinbeck as an outgrowth of the dehumanizing effects of capitalism and the way it sets people against one another.