You may do this by modifying the windows shortcut properties for Half-Life OpFor and adding "-dev -console -game gearbox" to the end of the "Target" (or file to run).

The following commands are available in console:

IMPULSE 101 - Give all weapons and ammo.

/GOD - god mode.

/NOCLIP - walk through walls/fly.

/NO TARGET - enemies ignore you.

/MAP xxxx - go to map xxxx.

/GIVE xxxx - give item xxxx.

/GIVE WEAPON_X - give weapon X, where X equals:

GRAPPLE
KNIFE
PIPEWRENCH
EAGLE
M249
SNIPERRIFLE
DISPLACER
SHOCKRIFLE
SPORELAUNCHER

/GIVE AMMO_556 (Ammo for M249)
/GIVE AMMO_762 (Ammo for Sniper Rifle)

Hidden Audio
Approach the Boss of Opposing Force to hear backwards audio composed of the phrase "To win the game you must kill me Randy Pitchford" when played correctly. Randy Pitchford is the leader of the development team at Gearbox Software.

Map List (for the /MAP xxxx cheat)
ofboot0, ofboot1, ofboot2, ofboot3, ofboot4, of0a0, of1a1, of1a2, of1a3, of1a4, of1a4b, of1a5, of2a1, of2a1b, of2a2, of2a3, of2a4, of2a5, of2a5b, of3a1, of3a2, of3a3, of3a4, of3a5, of3a6, of4a1, of4a2, of4a3, of4a4, of4a5, of5a1, of5a2, of5a3, of5a4, of6a1, of6a2, of6a3, of6a4, of6a4b, of6a5, of7a0