Hex Cheats:
Step 1: Editing your persist.dat file or downloading edited file

To enable cheats, you'll need to edit your persist.dat file, which can be found in the root of your Oni directory (usually /program files/oni/). To start, open the file with a hex editor. If you don't have a hex editor, you can find one at a site like Download.com.

For PC users, go to hexadecimal Offset 44 --make sure it's hexadecimal 44, not just decimal 44. Change the offset to "07" (it'll probably be "02" or "06" or something to begin with).

Save the file and proceed to Step 2.

If this is too tricky (as it likely is for most people), just download this pre-edited file for PC users and place it in your root Oni directory. This file will also allow you to start the game at any of the save game points.

The pre-edited file for PC users may be found at http://dl.fileplanet.com/dl/dl.asp?3dactionplanet/oni/persist.dat.

Step 2: Activate the Cheats
Once in the game, press F1. This will bring up the Diary screen. Type in the cheat you want from the list below to activate it. Typing in the cheat again from the Diary screen will turn that cheat off.

Cheat Codes:
shapeshifter - Change Characters (with F8 key)
liveforever - God mode
touchofdeath - Kill enemies in one shot
canttouchthis - Can't be knocked down or knocked back
fatloot - Gives you all kinds of ammo and health
glassworld - Makes most objects breakable
winlevel - Instantly Win Level
loselevel - Instantly Lose Level
bighead - Big Head Mode
minime - Mini Mode
superammo - Super Ammo Mode
reservoirdogs - Last Man Standing Mode (AIs fight with each other)
roughjustice - Gatling Guns Mode
chenille - Daodan Power Mode (you do more damage)
behemoth - Makes you giant
elderrune - Regenerate health over time
moonshadow - Phase Cloak
munitionfrenzy - Creates pile of all weapons
fistsoflegend - Fists Of Legend Mode
killmequick - Enemies are super tough
carousel - Slow Motion Mode
thedayismine - Developer Mode

Step 3: Developer Mode Commands

While in developer mode, you can use certain key combinations and access the command console (in dev mode):

Cntl-Shift-Y = FPS Counter (works always, not only in dev)
F7 = Cycle through all possible weapons
F8 = Change character
F9 = Start recording
F10 = Stop recording
F11 = Playback recording
Cntl-Shift-G = toggles between slow motion and normal modes
Cntl-Shift-S = toggles textures on some objects
Cntl-Shift-B = shows your current opponent logic
` = brings up the cmd prompt

Command prompt is perhaps the most interesting thing out there. You can execute script commands from there or assign values to internal variables. The best way to find out what commands/variable are there is to type at the cmd prompt: dump_docs. This command will dump all known docs and variables (sometimes with comments!) to the text file. CMD prompt has autocompletion feature, to accept the suggestion it gives you, use Tab key. Up and down arrows scrolls through the command history.

Some handy commands:
door_ignore_locks = 1 unlocks all doors for you ai2_kill kills all nearby AI chr_nocollision 0 1 'noclip' mode, allows you to walk through avoid jumping. The designed levels are beautiful.

More stuff can be found in the docs file created by dump_docs command.