FFXII - Walkthrough - Ogir-Yensa Sandsea
Platform 1 - East Tanks: no room for confusion here. Walk up the stairs to the first tank, through the 2nd and 3rd, to the next area.
Platform 1 - Refinery: Cutscenes, and Vossler joins the party as a guest. At the first oil platform, take the stairs down to the ground; head west, up the oil platform; it's a dead end, but the map for the Ogir-Yensa Sandsa is located in an urn here. Now go back down to the ground, and head north, pass through that area/platform, and head west to another oil platform. Take the southern path (southwest, to be precise) and move through to the next area.
Now that you have the map, you no longer need explicit instructions on where to proceed. You'll want to take the appropriate path so you arrive as "Yensa Border Tunnel", the more southern of the two ways to get to the Nam-Yensa Sandsea. In case you still need instructions:
Platform 1 - South Tanks: Go west at first oil platform; west at second oil platform; southwest at 3rd oil platform, to next area.
South Tank Approach: Can't get lost here. Head down the stairs and west to the next area.
Yensa Border Tunnel: a nice Teleport Crystal for all your trouble, and a seller of goods if you need some. On to the Nam-Yensa Sandsea.