In the books
In the A Song of Ice and Fire novels, Orkmont was once the seat of House Hoare when they were Kings of the Iron Islands. However when House Hoare went extinct during the War of Conquest, the Targaryens allowed the ironborn to chose which of their Houses would rule over them under the Targaryens. House Greyjoy of Pyke was chosen, so the ruling seat of the Isles was transferred from Orkmont to Pyke.
Orkmont is described as rugged and mountainous - possibly meaning that it is even more mountainous than Great Wyk, the largest of the islands. Orkmont is actually wealthier than Pyke, though not as wealthy as Harlaw and Great Wyk. Along with these other three islands in the chain, Orkmont's mountains contain mines that produce abundant supplies of iron, lead, and tin.
Orkmont is also, more or less, located in the middle of the Iron Islands: Harlaw is to the east, Great Wyk and Old Wyk to the west, Blacktyde to the north, and Pyke to the south (Saltcliffe is west of Pyke).