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*Assigning Enter to Activate inside Oblivion will allow it to be used for menu selections, item selections from containers, and activation without conflict, thus requiring only Enter to be assigned to the A Button, along with Left Click for speech skipping.
*You may find it more usable to assign the left and right arrows to the bumper buttons (rather than assigning all 4 to the left joystick) because the bumpers are not otherwise used during menus, and will be less likely to leave you accidentally changing back and forth between pages while trying to scroll a list in your inventory.
*If you don't mind walking for the fraction of a second you're putting your weapon away, the X button can also be mapped to Shift to enable placing map markers or removing items from your inventory (X+A). With the above arrow key binding suggestion, mapping Shift to the X button would also enable you to quickly toggle inventory when looking in a container (X+left/right bumper).