There's something of a discrepancy between the various maps concerning the size of that grassland area that Ommok conquered. In some it appears pretty small but in others it's much larger. Since it's been more or less empty and unused up until now, there's nothing stopping us from deciding that it's a rather small area, but should we choose to say that the recent maps portray its size accurately then I'll just write off Ommok's rapid conquest as possible due to the region being sparsely populated. It's easy to "own" a gigantic grassland when there's only a bunch of Hain tribes scattered around to contest your rule. Conquer their villages and you control the whole place.
Here is the recent map, with the extent of the firmly ogre controlled territory marked by the red slashes. Note that they control the hilly southeastern borders of the Venomweald (the map incorrectly calls those borders swampland) and not the inner Venomweald. Though ogres might venture into the Venomweald on occasion, it's still far too savage for even them to establish permanent settlements there.
The orange slashes represent the Hain lands that Ommok has conquered conquered, whereas the orange arrows are possible routes for expansion (read: ogre invasion).
Thus far Ommok has encountered virtually no resistance and is Galbar's equivalent of Alexander the Great. We'll see if that continues once he inevitably encounters the neighboring civilizations.