Start Half-Life using "hl.exe -dev -console -game gearbox.
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).
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.
Hidden Suicide Technique!
At the part where you see Gordon Freeman jumping in the portal to Xen, notice that the portal is still open for about 5 or 10 seconds. When you open the door, quickly run after Freeman and jump into the portal. Instead of landing on a platform, you will just fall and then you will see a short sequence showing how you died.
Third Person
Firstly, activate the console and bring it up during gameplay. Then, type in /thirdperson and press enter. You can now play Opposing Force in the third person.