Who? If you don't know Oliver offhand, it's cool—his name only shows up a few times in the book. But as Lily and Lonnie's father, he still plays a significant role in where his family has ended up since he and Marigold divorced.
We don't know much about Oliver except that he was kind of a hippie and Marigold married him because of his amazing Technicolor dreamcoat. Seriously. "He had this lovely coat," Marigold reminisces to Lily. "It had the most beautiful colors, colors I'd never seen before" (13.4). Oliver might have been a sharp-dressed man (and you know what they say about them), but his ill-fated marriage to Marigold is proof that you can't judge a guy by his coat.
While Lily feels estranged from him and struggles with just calling him "Dad," Oliver definitely left his mark on Lonnie: "Lonnie always told people he couldn't remember his dad, and this was true, except occasionally brilliant little pictures would surface in his mind, pictures framed with feelings" (30.27). In particular, he recalls a day at the beach with his father carrying him into the surf, telling him not to be afraid.
Trouble is, though, that when Oliver left Lonnie's mother, he took away the sense of security in this beach memory. It's possible that a lot of Lonnie's issues with stability come from having his father uprooted from his childhood. For more on this, see Lonnie's analysis elsewhere in this section.
But what's this? The book ends with Oliver reaching out from his new home in New Hampshire to call his family. While it's unclear what role he'll play in the Samson family from here on out, the fact that he calls them on this particular weekend of their perfect day seems to suggest that he wants to reconnect. We can't know for sure, but Lonnie may be finding a new connection with his lost father just as he's found a fiancée.