Orogs are descended from the Skullbiter Tribe of Mountain orcs who lived in the Spine of the World mountain range millenia ago. When they, along with other Mountain orcs, invaded Netheril in -3605 DR, they were cut off from their retreat route and cornered between the elven and Netherese armies. They sought refuge in a narrow cave at the end of a valley which, unbeknownst to them, led to the Underdark. They became lost but felt safe in the knowledge that they would not be followed by their enemies, delving further underground whenever they could. Eventually they came across a veritable jungle of luminous fungi and decided to settle there. They bred in great numbers and expanded into many surrounding caverns like a virulent plague. They subjugated the creatures already living there and eventually split into a dozen different tribes, discovering rich veins of metal and ore.
The collapse of the elven empires led to the orogs venturing back to the surface in large numbers, bullying their Mountain orc cousins into subservience and arming them for war with weapons made from Underdark metals.