Online communities are important spaces for members of marginalized groups to organize and support one another. To better understand the experiences of fat people -- a group whose marginalization often goes unrecognized -- in online communities, we conducted 12 semi-structured interviews with fat people. Our participants leveraged online communities to engage in consciousness raising around fat identity, learning to locate "the problem of being fat" not within themselves or their own bodies but rather in the oppressive design of the society around them. Participants were then able to use these communities to mitigate everyday experiences of anti-fatness, such as navigating hostile healthcare systems. However, to access these benefits, our participants had to navigate myriad sociotechnical harms, ranging from harassment to discriminatory algorithms. In light of these findings, we suggest that researchers and designers of online communities support selective fat visibility, consider fat people in the design of content moderation systems, and investigate algorithmic discrimination toward fat people. More broadly, we call on researchers and designers to contend with the social and material realities of fat experience, as opposed to the prevailing paradigm of treating fat people as problems to be solved in-and-of-themselves. This requires recognizing fat people as a marginalized social group and actively confronting anti-fatness as it is embedded in the design of technology.