Fall back to the Sephora (part 2) | Mission 1: Distress A: CM Guide

As soon as you receive info about unlocking access to the hatch, back out of the hangar (don't forget to take the armor, if you haven't already); this is the only way to stop the advance of the aliens. Again, head for the tunnel connecting Sulaco to Sephora. You'll watch and interesting cutscene there, and when it ends, you'll resume the game in the company of O'Neal.