Instructions
Wait until Ames tells you to equip your AK rifle. Ordinarily, you would do so to prevent the approaching Revolver Ocelot from realizing that you are not a real terrorist. Instead, equip your tranquilizer gun.
Shoot Ocelot to knock him unconscious, then run over to his body. Drag it briefly and put it down to shake his dog tag loose and collect it.
Call Rose to save the game, then return to the game. The game tells you that you failed the mission and forces you to repeat the sequence in which you find and talk to Ames, but if you look at your inventory, you will see that you still have Ocelot's dog tag.