Remember folks that yakuza "warnings" tend to involve violence. Going to "warn Ozu" probably means beating him (at the very least) as a lesson for messing with them.
I find stupid that these guys know that Yoshino is THE daughter of THE Yakusa in the region and they simply resolved to settle a little grudge against her, Kirishima I understand but her, I am impressed that they are alive until now.
@alacaelum must be because the info about her haven't reached Tokyo. Remember that a few chapters ago, nobody knows she's related to the Yakuza in the school she's in at Tokyo. Even much so if their groups have a cold war before this. It might be also possible that her grandfather kept the information about her from leaving the region. Her father died in an accident as stated before in a few chapters so it might be that reason.
Information about her being related to Yakuza seems to be kept extremely hidden. Even Kirishima worked hard to keep her identity from reaching other factions in their group. The reason for this might be because he had made a promise to her grandfather to keep her safe.