Half-Life: Opposing Force/We are Pulling Out

The tram
The tram ride in this section, unlike the tram rides of Freeman and Calhoun, is short and will have an enemy appear and attack you. Shortly after this single encounter, you will approach a platform, where you disembark.

A new contact
Exit the platform. By the time you reach the Glock 17, two vortigaunts will teleport from the tram and attack; first will appear on the platform, and the other on top of the tram. (note- its best to have shotgun readied and not pick up the glock until you kill the vortigaunts, as the glock deals less damage)

Head around the corner, you will find a overweight security guard trying to steal from a vending machine, who then asks you for money. his name is otis and its recommended to keep your wallet to yourself. have Otis follow you through the corridor, killing the headcrabs, zombies and hound eyes. When you see Barney and a scientist talking, a new alien will appear and kill both of them. Jump through the window, and open the door for Otis. (note that otis is a dipshit and will shoot you if you stand inbetween him and enemies)

Otis will unlock a door containing a few grenades for an MP5 you'll pick up shortly.

Go on the elevator and press the button to raise it. You will hear gunfire from allied soldiers, but they will get killed. Dispatch the one or two enemies at the top of the lift, and recharge the PCV.

Evacuation
Head through the vent above the HEV charger, and climb the ladder to the top. You will reach a military rally point, where a soldier uses a radio, and another tried beating information from a scientist.

The closet containing the scientist and soldier contains a few HEV charge cells. You can collect them either by going through a raised wall vent on the floor below, or by committing friendly-fire by killing the soldier. In either case, proceed down the stairs to the helicopter.

However, the helicopter access is closed and it takes off without you. Instead, the only route out will be through a wall which an alien tries to burst through. Proceed into this hole to start the next chapter.