Every major scene in the final movie, bar Ash, is in O'Bannon's script, [read the final 'battle' in both scripts, the wording is almost exact - this happens numerously throughout the scripts]. Every character is there [again, albeit Ash], albeit with a different name and different lines [Giler said they excised all of his lines - false, they re-worded a lot of it and switched the characters speaking them around]. Ripley is not so far removed from Ripley - he's the cautious one who won't allow anybody to break the rules, he won't allow the Dallas character back on the ship when the Kane character is infected, etc. The Parker character is still a man more concerned with his end of the bargain, and on, [I think Giler and Hill rounded out Melkonis/Lambert a lot more, or at least shaped her character].
Kudos to Giler and Hill for Ash and the Company, though their script was also pulled back a little - in their draft, the Company created the Alien themselves, there was no real, truly 'alien' presence in the movie at all. Further drafts excised this [actually, it would have been dropped due to budget, but Scott was focused on showing the pyramid, not the Company Cylinder. O'Bannon recalled that when Scott read the original script he wanted to go back to the original idea ie pyramid. But the budget and time concerns nixed that idea].
Ideally, the credit should include all of their names. According to O'Bannon on the Anthology, he expressed the idea that it should, but Hill was dismissive. The two producers don't help themselves much by being constantly slanderous towards the guy, whereas O'Bannon praises those around him that helped out with the story [Shusett, Cobb]. I can see why he'd hold a grudge towards the two. I probably would. And of course, Giler and Hill pulled the same thing on Cameron with his Aliens treatment, bumping his name to third and taking all of the money for it: Walter and David got a check for my treatment, and I got nothing. I was pretty pissed off about that one - James Cameron.
It's a dirty biz'ness.