Mion and Shion's parents live there, and it is where the Okinomiya Police Department is located, which are responsible for investigating any crimes that may happen in Hinamizawa.
Compared to Hinamizawa, Okinomiya has many more modern facilities and businesses. It contains a toy store, the restaurant Angel Mort, a library, and an apartment complex where Shion lives alone. Her partment complex she lives in is also owned by the Sonozaki family, and so many of the tenants are shady people connected to him in some way or another. There is also an elementary school with much better resources and teachers than the Hinamizawa Branch School, but because of its distance most kids from Hinamizawa enrolled in the Branch School instead.
There is a district in Okinomiya called Hirasaka (平坂) that worships Oyashiro-sama as a god of friendship. It is theorized that Hirasaka's name used to be written differently (比良境) and was connected to Hinamizawa's past as Onigafuchi Village. The old Hirasaka was known as a place where the human and demon worlds intersected; Onigafuchi's residents feared visitors being possessed by demons if they traveled through Hirasaka, so they would make them use a wedge to cleanse their impurities when they entered and exited.