To enter these cheat codes, pause the game and hold down L1 + R1 then press O, O, Left, Triangle, O, X. You should now see the words "Enter Cheat".

Player Suicide (Single Player only): X, Triangle, O, Right, Triangle, O
Hide HUD: O, Down, O, Up, Triangle, O
Pickup OSS Document: Up, X, R2, R1, Up, Square
Disable Shellshock: L2, R1, L2, L1, Triangle, Triangle
Kill Nemesis: Down, L2, L1, R1, Up, Square
Unlimited Ammo: L2, L1, Square, L2, L1, R1
Full Adrenaline Meter: L1, R2, R1, Up, Triangle, Circle