O'Bannon was 63.
* Sorry, my error: O'Bannon wasn't involved with Aliens, though original Alien producer Walter Hill was. It is true, though, that Cameron was hired to write and direct the sequel even before Cameron made The Terminator: I've seen the story treatment, which is dated 1983.
There's a lot more I could've cited about O'Bannon, who was a jack of all trades.