Legion has many mysteries, and there's only a few episodes left to unravel them all. And while it's not the most pressing of the show's questions, what happened to Oliver Bird, Melanie's husband, on Legion? Like most things that happen on Legion, it may seem complicated because of how the information is dished out, but the explanation is actually pretty simple, despite the mind-bending visuals. Oliver, who helped build and found Summerland with his wife and Cary, clearly was gifted mechanically. Not only did he build that coffee machine that has the ability to tell fairy tales, but he also managed to figure out a way, through technology, to travel to the "astral plane," that void-like area that David traveled when his mind wandered under sedation.
The astral plane is a dimension where it seems characters' minds can go, but their bodies do not follow. David was under Cary's medical care while he was in the astral plane, while Oliver's body is in a frozen chamber in Summerland, with his body preserved in a suit that makes him looks like a turn of the century deep sea diver. The ice where his physical body is kept echoes the giant ice cube where Oliver's consciousness is trapped right now.
Legion takes place in an ambiguous time period, but from how Oliver acted and talked to David when they met up in the astral plane, Oliver became trapped sometime in the '60s or '70s, with his mentions of "free love," his beat poetry, and his bright yellow leisure suit. Certainly, the implication is that when he became trapped, he became unaware of the outside world, perhaps because he doesn't have psychic powers the way David does. However, Oliver does have some ability to navigate, because he did both beckon David inside his frozen home and, after a short chat, show him the door. Once David continued exploring, it seems that it was only the assistance of his "parasite," embodied by Lenny, that could help him escape.
As of "Chapter 5," Oliver, without the help of any psychic, is still trapped in the astral plane. But now, the rest of the characters are trapped as well, in a version of Clockworks run by Lenny, suggesting this is either all in David's mind... or, perhaps, in the same plane where Oliver is being held. So far, Melanie has become convinced that Oliver could eventually be led back to his body, and maybe her long-held devotion to him will pay off with a moment when they're finally able to reconnect while both trapped. There's no guarantee that Oliver's fate will pay off by the end of Legion Season 1, but he could be a factor in how the characters are finally able to escape David's mental prison.