As I understand it, yes. The hexes in the immediate vicinity of Oleg's are plains, while further south you'll find the hills terrain (like around the Stag Lord's Fort).
I want a clarification about which hexes represent which terrain types. The mountains are extremely clear, and the hexes that are like the ones in Thousand Voices in the Glenebon Uplands would be forests, while the ones in the Hooktongue Slough would be swamps. That leaves the yellow-brownish terrain and the greenish terrain. I take the yellow-brownish terrain to be hills and the greenish terrain to be plains. Am I correct on all accounts?
You are correct on all accounts.
Per James Jacobs post, somewhere on the boards (I just copied it to my Kingmaker cheat sheets):
The terrain types are not labeled. The light green is plains, the medium green with the bumps in it is hills, and the dark green is forest.
The plains are pretty open, with grass and isolated copses of trees.
The hills, on the other hand, have more trees. They're not totally covered, but trees should be pretty common in the hills. They're just not so dense that they're non-stop forest.
The Gazetteer section "Into the Wild" on page 56 of Stolen Land has some physical descriptions of the terrain itself and a featureless topo map on pages 58 and 59 if you want to compare terrain and get a better feel for things.
And as a side note, the hilly section of the Greenbelt? The Kamelands [Kaym-lands]?
Wikipedia has some great reference pics of the namesake terrain.