Oliver and Susan actually get along with each other quite well. She decides to take him to Big Pond, a nearby scenic spot, with her sister Bessie and her husband, John.
At some point, Susan climbs up a cliff overlooking a waterfall and hangs over the edge so she can peer down. While she does this, Oliver holds on to her ankles so she won't fall. Lyman sees this as the moment when she falls in love with him.
At some point (and somehow), the pair decides to marry. Er, all right. Oliver's plan is to visit his parents in Connecticut for a few days before heading back out west to get things ready.
Augusta freaks out when she hears the news, accusing Susan of keeping secrets. It certainly doesn't help that she doesn't dig Oliver very much.
But, that doesn't end up mattering—in 1876, eight years after they first meet, Oliver and Susan are married.