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 text 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). For Mac users, you must change hex offset 47 from 02 to 07. 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 or this one for Mac userss 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.
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.
Step 3: Developer Mode Commands.
While in developer mode, you can use certain key combinations and access the command console. These are the keys I have found (in dev mode):
Cntl-Shift-Y = FPS Counter (works always, not only in dev)
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:
More stuff can be found in the docs file created by dump_docs command.