Well , unlike the whole cast , when oshino make a deduction after knowing all the current facts , i think it's pretty much safe to assume it is true ... Well him and another character ( that you'll meet later ) are ..unfortunatly always right , regardless of your perception of the events.
I'm not saying that the characters themselves are perfect , no .. Far from it .i'm saying that their deductions/conclusion are to be trusted at face value. Oshino perspective can make him have the right conclusion when he has all the facts , and in hanekawa case , he did have them all.
I really do not agree that Oshino is always right, even when he has all the facts and especially in the case of saying that Hanekawa was responsible for her legal guardians' abuse. The argument that her unusual behavior is the reason they beat her and deprived her of things like her own room is circular, because she only behaved that way because they beat her and deprived her of basic things like her own room. I'd sooner believe Oshino said it so that he could get Araragi to not see Hanekawa as perfect (as in, if Oshino says "Girl's evil" then Araragi will acknowledge her faults in the course of defending them, rather than disavowing her faults) than that Oshino Meme actually meant that a little girl was responsible for the abuse her legal guardians dished out on her.