I haven't seen anything in the tutorial or anywhere about how to use them and right now I'm dying of no oxygen with 3 oxygen tanks in my inventory.

Hi, these are the controls

Move - WASD / Left Stick
Aim - Right Mouse Button / Right Stick
Fire - Left Mouse Button / Left Trigger
Reload - R / X (xbox) / Square (playstation)
Sprint - Left Shift / Left Bumper
Action - E / A (xbox) / X (playstation)
Cancel - Q / B (xbox) / Circle (playstation)
Inventory - I / Y (xbox) / Triangle (playstation)
Items - Tab / Left Bumper
Flashlight - Z / Dpad Up
Change Weapon - Mouse Wheel / Dpad L/R
Pause - Esc / Start

Just press the action button to use the item :)