Get ready for a battle, because as soon as you open the large door leading outside you'll find yourself in the area where mercenaries are fighting Xenomorphs. It's a good idea to let the aliens deal with the soldiers so you won't waste ammo or take any unnecessary risks. It doesn't mean, though, that you shouldn't be careful. Watch out especially for Spitters; make sure to shoot them from a distance and not stay in one place for too long.
Keep moving forward, eliminating both the Weyland-Yutani mercenaries and Xenomorphs. Remember to use covers and look around for collectibles. When you get close to the above building, focus your fire on the stronger soldier using your best weapon available against him.
Depending on preferences, you can go through the aforementioned building (there should be some supplies to take) or stay on the main path. Regardless of the choice, expect to run into new enemies. Be extra careful around the area pictured above; apart from regular mercenaries, there's also a heavily armored opponent.
You can also take a look inside the second building, but there's nothing in there besides some supplies. Make sure, though, to check out the third garage; there is a Dog Tag lying on one of the crates (screenshot 2) (PFC Tim Crowe).
Leave the building and follow O'Neal. Expect new encounters after reaching the area illustrated above. In spite of what it looks like, you don't need to hurry to get behind the covers, because there are only Xenomorphs to eliminate. It's better to stay on the hill and take them out from a safe distance.
Once they're down, pick up collectibles and head for the entrance to the underground facility. Go through a small tunnel, receiving new objectives in the meantime.