Relationship: Ex-girlfriend - close friends / Estranged Relationship
Details: Theresa is a childhood friend of Ryan, and the two have engaged in an "off and on" relationship, which came to a halt upon Ryan's departure from Chino. Over the Thanksgiving holiday, Ryan briefly returned to Chino to pay off Trey's debts, at the request of a still incarcerated Trey. Ryan went to Theresa's house to pick up a car from her brother that would later be used to pay these debts. During this time, Theresa and Ryan each exchanged apologies about the lack of communication during Ryan's absence, most notably Ryan's failure to inform Theresa of his exodus from Chino.
Soon after Theresa and Ryan met again, when Theresa began working in Newport for unexplained reasons. Eventually, Theresa revealed that she and Eddie (a friend of Trey's from Chino) had become engaged. Soon after the engagement, Theresa developed cold feet (most likely due to the domestic violence she suffered at Eddie's hands). A short time before this, Marissa Cooper and Ryan had broken up over recent complications involving Oliver Trask. Naturally, Theresa and Ryan began a relationship soon after. Eddie persisted in his attempts to win Theresa back, even showing up at Harbor High School to confront Ryan about his suspected relationship with Theresa. This finally culminated in Eddie making an appearance at the congratulatory party for Caleb Nichol's being named Man of the Year. Eddie and Ryan came to blows, with Eddie being dragged away from the party and Ryan drenched in the pool. Theresa realized that she needed to resolve these issues, and left Newport to return to Chino and Eddie. Theresa does not appear again until the end of season two, in The O.Sea, where she convinces Ryan to trust Marissa's explanation about what happened between her and Trey. We learn at the end of this encounter that she has the baby, is living with her mother, who informs us that the baby "looks more and more like his daddy every day". She informs Sandy of this turn of events, who tells her not to tell Ryan, as he will no doubt interrupt his college visit at Berkeley. She does tell Ryan, however, who lives up to Sandy's expectations, and returns home. Theresa comes to visit him, and informs him that the baby is in fact not his, but Eddie's. (However, it's strongly suggested that the baby is in fact Ryan's since it is seen with all light features like the young guy). The two spark another of their "relationships" for a short time, but Ryan's fight with Kevin Volchok convinces her that Ryan has not yet matured enough for a relationship with her, and fears the influence Ryan might have on her baby.
Details:Ryan Atwood is the second official boyfriend of Marissa in the series. Although extremely on-and-off- throughout the course of the series - their relationship originated from a Platonic friendship in the pilot episode, which subsequently turned into romantic feelings. From the tenth episode of the first season, the couple officially started their romantic relationship. The beginning of this relationship sees its first end when Marissa feels that Ryan has exaggerated with the friendship between her and Oliver Trask. The second course of their relationship fails due to Theresa Diaz and their illegitimate child, which Ryan agrees to help grow - returning to Theresa's arms and letting Marissa return to her drinking habits. The third attempt of the relationship between Marissa and Ryan ends after the suicide of Johnny Harper where to cry and need a break from each other. The last attempt of a couple's relationship ends when Marissa is killed by a drunken Kevin Volchok who sends Ryan and Marissa's car out of the way, killing the girl instantly.
Relationship: Exes / friends /they still have feeling for each other
Details: Ryan and Taylor Townsend's relationship begins in season 4, when Taylor decides to help Ryan solve his sleep problem. He and Taylor begin to spend more and more time together, and begin to develop feelings for each other. Ryan is still skeptical, because in addition to never dating a girl like Taylor, he has lost his first love : Marissa Cooper, a few months back. Ryan has not yet gotten through the mourning, despite everything though, he still decides to start a relationship with the girl. Thanks to Taylor's personality, Ryan will be able to start living and laughing again. At the end of the series, the status of their romantic relationship is left unclear, the two anyway, are seen smiling at each other at Seth & Summer's wedding.