Start the game with the command-line parameter "+set thereisacow 1337 +set developer 1 +set sv_cheats 1 +set monkeytoy 0" to activate the cheat codes. While playing, press ` to bring up the console, then type any of the following codes:

Example: "C:\Call of Duty\CoDUOSP.exe" +set thereisacow 1337 +set developer 1 +set sv_cheats 1 +set monkeytoy 0

Code - Result:

give (item name) - Spawn indicated item
give health - Full health
give all - Get all items
give ammo - Ammunition
god - Invincibility
notarget - Ignored by enemy
noclip - No clipping mode
debug (0-1) - Toggle debug mode
developer (0-1) - Toggle developer mode
ufo - Fly mode
jumptonode - Teleport to a specific node
map (map name) - Play indicated map
kill - Suicide
cvarlist - List all cvars
cvardump - Dump all cvars to console
cvar_restart - Reset all cvars
cmdlist - List all console commands
shaderlist - List all shaders of current map
imagelist - List all images of current map
snd_list - List all sounds currently used
entitylist - List all entities currently used
bindlist - List all currently bound keys
gfxinfo - View graphics information
savegame - Save game
loadgame - Load saved game
reset (variable name) - Reset variable to default value
cinematic - Play a cinematic file
cg_crosshairAlpha (number) - Set crosshair transparency
g_gravity (number) - Set gravity value; default 800
cg_hudAlpha (number) - Set HUD transparency
ai_corpsecount (number) - Set seconds that dead bodies remain
cg_shellshock (number) - Set shellshock duration
g_gameSkill (number) - Set skill level
g_knockback (number) - Set weapon knockback; default 1000
cg_showMiss (0-1) - Show miss distance when hitting NPCs
testmodel (model name) - Spawn indicated model
cl_paused 2 - Free movement while paused
cl_paused 0 - Return to normal paused screen
timescale (number) - Set game speed; 1 is default
g_ai (0 or 1) - Toggle AI
g_debugBullets (0 or 1) - Toggle bullet debug mode
cg_marks (0 or 1) - Toggle bullet marks
con_debug (0 or 1) - Toggle console debugging.
cg_drawCrosshair (0 or 1) - Toggle crosshair
cg_brass (0 or 1) - Toggle ejecting shells
g_drawEntBBoxes v - Toggle entity bounding boxes
r_fog (0 or 1) - Toggle fog
cg_drawFPS (0 or 1) - Toggle framerate display
fx_draw (0 or 1) - Toggle FX
fx_debug (0 or 1) - Toggle FX debugging.
fx_freeze (0 or 1) - Toggle FX freezing
fx_enable (0 or 1) - Toggle fx. If set to 0, fx disabled.
cg_drawTimer (0 or 1) - Toggle game timer
g_debugShowHit (0 or 1) - Toggle hit debug mode
cg_drawStatus (0 or 1) - Toggle HUD
cg_draw2D (0 or 1) - Toggle HUD and crosshair
cg_letterbox (0 or 1) - Toggle letterbox format
g_debugDamage (0 or 1) - Toggle NPC health display
r_drawWorld (0 or 1) - Toggle objects and backgrounds
r_drawEntities (0 or 1) - Toggle objects and NPCs
g_debugMove (0 or 1) - Toggle player and AI movement debug
cl_debugMove (0 or 1) - Toggle player and AI movement debug
cg_noRender (0 or 1) - Toggle rendring
cg_shadows (0 or 1) - Toggle shadows.
g_spawnai (0 or 1) - Toggle spawning
cg_subtitles (0 or 1) - Toggle subtitles
cg_thirdPerson (0 or 1) - Toggle third person view
g_vehicleDebug (0 or 1) - Toggle vehicle debug mode
r_showtris (0 or 1) - Toggle wireframe mode