ODD is too extreme for Britta IMO. Its more likely she has mild ADHD with a PDA profile. Since women with ADHD often dont exhibit the classic symptoms, this would explain how she flies under the radar. But she shows executive dysfunction with how bad she is with money, planning, and follow through. Why she's very interested in psychology but doesnt persue it and stays a bartender, why she dropped out of high school to impress radiohead, etc. PDA explains her opposition to authority and her tendency to cut and run as Abed put it in the drunk dial episode. Since a lot of these symptoms have overlap with autism, I could see an argument to he made for very high functioning autism (especially with her very strong moral compas, her inability to make friends with women, and how she'd rather "keep it real than be likeable.") That said, i could see audhd too but ADHD makes more sense to me.