Once Human codes are one of the ways you can claim in-game items for free, as well as the Energy Link currency you use to do just about everything in the new open world survival game. Unfortunately, these codes don't seem to include any of the game's premium currency of Crystgin for cosmetics.
However, if the motorcycle vehicle skin is anything to go by, it seems like codes themselves will sometimes include cosmetics you can claim. I'll lay out the most recent Once Human codes below as well as provide info about how to redeem them. There aren't any expired codes currently, but there is one slightly unusual code that comes in seven different variants.
As well as the above you can redeem one of the following influencer codes below. They all give the same reward; 300 Energy Link, two Activators, and one Adrenaline Shot:
It's pretty easy to redeem Once Human codes, though you'll need to complete the initial prologue quest, fight the Siren, and enter the main open world before you'll be able to. Once you've done all that, you can redeem codes in-game via these instructions:
Since code rewards don't go to the mailbox in Once Human, you'll get them as soon as you redeem a code.