Oil leak between oil filter and engine block

The part is the oil cooler. The nut is removable. You'll need to replace the O-ring between the cooler and the block, the sealing ring on the bolt (if equipped) and I would replace the coolant hoses to the cooler too. Here is a pic: http://img45.imageshack.us/img45/8453/13ss00a05xf7.gif #18 Oil Cooler, #19 bolt, #33 o-ring