The Northside Territory was a territory on the planet Sigma Iotia II. It was controlled by Boss Bela Okmyx, who boasted that it was the biggest in the world.
However, according to Okmyx, his outfit was always under attack. During the 2260s, Okmyx people were being hit by men from Jojo Krako's Southside Territory. Krako also had Okmyx' communications bugged.
Some in the territory were dissatisfied with how well the boss took care of them. For example, in 2268 one woman complained that no laundry pickup truck had come by for three weeks. She also felt unsafe due to the crumminess of the street lights. She claimed to have tried to sent Okmyx a letter on the subject, which he had sent back with postage due. (TOS: "A Piece of the Action")
It is never revealed exactly how large Okmyx's territory actually is, and indeed the episode is written as if Okmyx and Krako are both within a single city, as evident by Kirk and Spock driving between the two mob headquarters in a classic automobile in a relatively short amount of time.