There isn't a distinction between offline and online gameplay for Metal Gear Solid V: The Phantom Pain, as such there are not any cheat codes as changes in the single-player mode would affect multiplayer mode.
There is one feature that makes the game significantly easier - the chicken hat. This snazzy piece of headgear makes it so the first three soldiers who see you break down in fits of laughter and are vulnerable to attacks. It is acquired after failing a missing several times, around five failures, and can be equipped from the game options menu.
If you continue to fail the mission, I believe ten failures total, you'll be granted the baby chicken hat. This egg-shaped apparel prevents all enemy soldiers from fighting Big Boss, as they can't ever take you seriously. While the enemy soldiers are doubled-over laughing at you feel free to peruse their base, fulton extract freely or anything else that comes to mind.
Using either piece of headgear makes the game significantly easier, so much so I'd consider it a form of cheating. Let it be known that wearing either hat limits your mission score to rank B or below.
You can read a little more about MGSV:TTP cheats here (it's mostly Easter eggs though)
The headgear, in all it's glory:
And the lil' chicken hat too:
