Indeed, oil probably will not deactivate the primer. If you think about how primers work then you will understand that there is little to no danger decapping a live primer.
When a gun fires the cartridge, the firing pin is hit with a sudden and fast blow which transmits a sudden and fast blow to the primer which sets it off. Make no sudden or fast movements with the firing pin in your decapping die (sic) and it will not set off the primer. Nice and easy.