Oten City is a city neighboring Daten City. The city has a tall structure in its middle, which is where Hell's Gate is located.
The city is first mentioned in the season 1 finale, "Bitch Girls: 2 Bitch". After Stocking slices Panty up into 666 pieces, Corset explains that in order to get Panty back, Brief would have to collect all of Panty's pieces while traveling to Oten City. Once he unlocks Hell's gate using his penis, Panty would be revived.
In the lone chapter of the Cemetery Hills doujinshi, "Geekboy Homecoming", Brief and Chuck try to find all of the missing pieces of Panty in the city. They succeed, but are caught off-guard by a muscular Corset, who surprises the two by hiding in the structure and unlocks the gate.