This is pretty basic, just bind the following code:
scr_oldschool "1"
Example: bind BUTTON_BACK "scr_oldschool 1"
Also, for this and the following to work, you have to bind this aswell:
'fast_restart'
Example: bind DPAD_DOWN 'fast_restart'
Tutorial:
Once you've bound the two codes, find a match and make sure you're host, when the game starts, choose a class, press the button you bound for 'scr_oldschool 1' then press the button for 'fast_reload'.
The game will now be Oldschool.
ALSO:
You can also replace that, or make a different bind, for
HARDCORE - scr_hardcore 1
NO KILLCAM - scr_game_allowkillcam 0
HEADSHOTS ONLY - scr_game_onlyheadshots 1
Have fun.
