Wait for O'Neal to open the maintenance hatch, then jump down, crouching when necessary. In the new room, move towards the only available door. Wait until you're quarantined and proceed forward. Listen to a new conversation in the next room, then turn towards the newly unlocked interactive door.
The current part of exploring the USS Sulaco is very linear, and what's more, you won't run into any aliens for now. The fighting will resume only shortly after using the above entrance to the gravitational well.
Start by attacking the Xenos on the right. Use the pulse rifle or any other weapon with similar accuracy, taking them down before they come closer and jump down on the platform you're currently on. After eliminating the first wave, go left. Get ready to send some other Xenos to hell. Here as well it's better to shoot them from a larger distance and assist yourself with the flashlight to spot them in less lit places.
Stop when you reach a closed door. Theoretically, you could use the welding torch yourself, but this way you'd become defenseless and your teammate would have a hard time trying to keep you alive. It's much better to wait for O'Neal and let him handle the door, protecting him in the meantime (I recommend the shotgun).
Start examining the new linear corridors. After entering the control room, clear the area of Xenomorphs, then look around closely for an Audio Log, pictured on the screenshot. Once you've thoroughly checked out the whole room, move to the entrance by which O'Neal should already be standing.
In the next room, you'll be surprised by several Xenomorphs, so be ready to use your weapons quickly. Continue the move forward; there is another Dog Tag by the computers (PFC M. Moore). Take the passage leading to the second level of the gravitational well.
Again, there will be Xenomorphs coming at you from around the area, which they better be kept at bay. Nevertheless, keep going forward. Watch out for the aliens that will quite unexpectedly use the door on the right at the very end of the metal gallery.
Take the newly unlocked passage and get ready to eliminate the aliens that will show up on the ceiling. Check out every adjoining corridor and room; most of all, you'll find a complete set of armor as well as Hick's Shotgun, pictured above, which is a Legendary Weapon. Replace your regular shotgun with it.
Proceed to the passage leading to the metal galleries on the third level of the gravitational well (the above screenshot). Attack other Xenomorphs and get to the new locked door.