Nailing/stapling is the fastest way, and is how a lot of people will attach standard OSB sheathing, but nothing beats the triple coated construction screws.
For 2x planking, I would think screws would be the way to go. in 8 inches, I would think 3 would be fine. We use 6 staples in feet of OSB, which comes out to a staple about every 8 inches. You could just use 2 per board, but 1 in the middle may help reduce its tendency to cup.
The best screws I have found are triple coated construction screws, the ones with the shiny gold-colored coating on them. Torx head is the way to go, with an impact driver.