One Piece Chapter 28, page 11: The Crescent Moon

Back at the southern slope, Usopp and the Straw Hats have finished their preparations. Usopp announces, "OKAY! WE'RE DONE! WE'VE SPRAYED A MASSIVE AMOUNT OF OIL HERE! NOW THOSE BASTARDS WON'T BE ABLE TO GET UP THIS SLOPE." He emphasizes, "THIS IS THE MOST EFFECTIVE WAY TO BATTLE THEM! NO MATTER WHAT, WE MUST GUARD THIS SLOPE WITH OUR LIVES." Nami warns him, "JUST DON'T FALL DOWN THERE YOURSELF! OR ELSE YOU'RE SCREWED!" Luffy cheerfully agrees, "YEPP!"