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This might not be what you're looking for, but that sounds like they're just sampling from more folksy stuff and rapping to it. You'll find lots of mash-ups that attempt sounds like that. I can't seem to find fantastic examples on youtube, unfortunately. You'd probably mostly have to comb mash-up sites.
I haven't listened to much K-Os, just that one song about crabs As Darkewolfe mentioned there are many mashups combining Hip-Hop with Folk/Blues, but I personally suggest finding stuff that was written specifically for the beat, such as this old Wise Eyez track that sampled House of the Rising Sun by Bob Dylan.
Posts
Something slower
And something more upbeat
http://www.youtube.com/watch?v=0cFmfsyQs_E
http://www.youtube.com/watch?v=BOLczfWMXDM
I fucking love K'naan and have both his records. But I'm looking for less mashups though... I want to be able to see something like this at a concert.