Oscar Nero moved to Port Charles in 2017 and struck up a friendship with fellow classmate Josslyn Jacks. One night in June, Oscar and a group of kids showed up at Josslyn's house for a party while her mother was gone. Josslyn had a crush on Oscar and was excited that he was there, but her joy was short-lived because Bobbie arrived and broke up the party. Carly was furious when her mother reported that Josslyn had been drinking, so she grounded Josslyn, but Josslyn rebelled. Josslyn repeatedly sneaked out to meet Oscar, and they'd often meet on the footbridge.

Carly tried a different tact. She met Oscar and gave her consent for Oscar and Josslyn to date. Carly quickly grew to like Oscar and was eager to meet his mother, but he explained that his mother was a single parent and worked long hours. Oscar and Josslyn's relationship blossomed, and they shared their first kiss on September 14, 2017.

Oscar opened up to Josslyn about his frustration over his mother's refusal to talk about his father. Oscar admitted that he didn't even know the man's name. Josslyn offered to help Oscar find his father, so they snooped around Kim and Oscar's apartment and uncovered a music CD that Oscar's father had made for Kim. Weeks later, Kim and Oscar attended Thanksgiving dinner at the Corinthos mansion.

Oscar was surprised when Kim recognized Jason Morgan then revealed that she'd known his identical twin brother, Andrew Cain, in San Diego. Oscar and Josslyn realized that Drew might be Oscar's father and decided to pay Drew a visit on the pretext of needing Sam's help with a school project. Josslyn managed to swipe the glass that Drew had been drinking from, so she and Oscar submitted Drew and Oscar's DNA to a company that ran paternity tests. The DNA test confirmed that Drew was Oscar's father.

Kim explained that she and Drew had had a brief affair that had ended when Drew had been deployed for a tour of duty overseas. Kim had lost touch with Drew, and when she had tried to reach out to him to tell him about their son, the Navy had informed her that Drew had deserted. Oscar was cautious about getting to know Drew because he was afraid of rejection, but Drew was patient. Oscar slowly got to know his new family, and he eventually turned to his father for help when school bullies threatened one of Oscar's friends. Drew taught Oscar self-defense moves at Volonino's gym.

Meanwhile, Oscar and Josslyn planned to host an alternative dance party on the same night as their school's dance because the school had decided to require that all students attending the dance dress according to their gender. Oscar and Josslyn objected because the new rule affected one of their friends, so their alternative dance was a protest. They moved ahead with their plans, knowing that they faced a possible suspension.

The party was a success, but disaster struck when an earthquake rocked Port Charles. Oscar and Josslyn became trapped in the walk-in freezer. They both suffered hypothermia, but Drew and Kim worked together to rescue the teens. Oscar fared worse than Josslyn because he had given her his jacket. Oscar took a little longer to recover, so Josslyn stopped by his apartment to check on Oscar every day until he was able to return to school. Oscar's relationship with his father also improved, and father and son grew closer. Oscar once shared with Drew a desire to visit the top of Mt. Kilimanjaro, and he encouraged Drew to explore options to recover Drew's lost memories.

When Drew was handed the key to regaining his memories, he decided not to pursue it because he didn't want to risk losing the few memories that he had of his son. Oscar took an apprentice job working for his father's media empire. Oscar also became acquainted with Josslyn's childhood friend, Cameron Webber, and he was not impressed.

On August 28, 2018, Oscar and Josslyn made plans to meet in the park to celebrate their one-year anniversary. Unfortunately, Oscar suffered a headache, became dizzy, and then passed out. He was taken to the hospital, and the incident was initially brushed off as low blood sugar. Josslyn felt bad because she had thought that Oscar had stood her up. She confessed that Cameron had kissed her. The boys exchanged words and punches, but Cameron walked away with newfound respect for Oscar.

A few weeks later, Cameron realized that the adults around him and Oscar had been hiding something. Cameron was stunned when he stumbled on the truth, and he went to Oscar with the shocking discovery. Oscar confronted his parents, who confirmed that it was true: Oscar was terminally ill with an inoperable brain tumor. Kim had known the truth since 2016, but she had kept the secret because she had wanted Oscar to have an opportunity to live a normal life without the burden of fearing that it would be cut short. However, there was hope; Kim had learned about an experimental treatment that might save Oscar.

Hurt and angry, Oscar decided to take control of his own destiny. He talked to Dr. Terry Randolph, his oncologist, and opted to forgo the treatment. Stunned, Kim plotted to force the treatment on her son, since he was a minor. Oscar rebelled. He enlisted Alexis Davis' help to divorce his parents. Alexis advised Oscar that he would have to prove that he could support himself, so Oscar dropped out of school and got a job at Charlie's Pub, bussing tables.

Finally, Daisy Kwan invited Oscar to move into the Dawn of Day house. DOD was an organization founded by life coach Shiloh Archer that helped local communities through outreach programs. Oscar also decided not to tell Josslyn about his diagnosis. Instead, Oscar broke up with her because he hoped to spare her the pain of watching him die. Cameron urged Oscar to reconsider, but Oscar insisted that it was for the best. Josslyn was brokenhearted, and she enlisted Cameron to help her make Oscar jealous. Oscar saw through the ploy, and he and Cameron began to bond as friends. Oscar also entrusted his uncle Jason Morgan with the secret.

Eventually, Cameron told Josslyn the truth. Josslyn raced to Oscar's side, and she made it clear that he could not chase her away. With Josslyn's support, Oscar made peace with his parents, and he agreed to undergo the experimental treatment.

On December 5, 2018, Oscar underwent the treatment. The side effects left him nauseous and weak. During his recovery, he asked his Uncle Jason to be the executor of Oscar's will. Jason was touched, and he agreed. Oscar also put together a bucket list, and he and Josslyn began to tick things off. Oscar's spirits improved, and he had reason to hope when scans showed that his brain tumor hadn't grown.

In March 2019, Oscar, Josslyn, Cameron, and Josslyn's best friend, Trina, decided to take a day adventure to Niagara Falls. Cameron had his driver's license, and Oscar had a car. Josslyn paid for the trip, using money from her "adventure fund." The teens set out for the trip, and they rented two motel rooms. Oscar and Josslyn had decided to make love for the first time, but Oscar suffered a terrible seizure and collapsed.

Oscar was rushed to the hospital, and his parents were called. Kim and Drew were not happy that Oscar had taken off, but they were too relieved that he had recovered from the seizure to lecture him. Everyone returned to Port Charles, and Oscar underwent more tests. The news was not good; Oscar's tumor had grown. Josslyn and Oscar were more determined than ever to make the most of the time they had left.

On April 4, Oscar and Josslyn spent an idyllic day together, and they ended up at their special footbridge. They talked about their failed attempt to get married at the courthouse earlier that day then Oscar surprised Josslyn with a padlock similar to the ones that people put on a bridge in Paris to symbolize their love. Oscar wanted to start the tradition on their footbridge. Josslyn was touched by the gesture, and she pulled a bottle of nail polish from her bag to paint their initials on the lock. They found the perfect spot then snapped the lock closed.

Moments later, Oscar suffered a seizure and collapsed. He briefly slipped into a coma, and when he woke up, the right side of his body had been weakened. Terry broke the news that Oscar's condition was grave. Oscar and Kim accepted Monica's invitation to move into the Quartermaine mansion so that Oscar would be well cared for and surrounded by family in his final days. Josslyn remained at Oscar's side, and together, they wrote a song that they hoped to perform at the upcoming Nurses Ball.

On May 28, 2019, Drew and Kim arranged to take their son's ashes to the top of Mt. Kilimanjaro. They thought it would be fitting for that to be his final resting place, since he had always dreamed of going. Monica supported the decision, but she asked Drew if he would mind if she put a plaque bearing her grandson's name in the family crypt, anyway. Drew was touched by the offer.

On July 10, 2019, loved ones gathered for the reading of Oscar's will. He bequeathed his car to Cameron, his keyboard to Ned for Leo Falconeri, a picture of Wiley Cooper Jones to Michael Corinthos, and a compass to Josslyn. He also left Monica a travel guide to a place that she had always wanted to visit, Drew received the cup that Oscar and Josslyn had used for the paternity test, and Kim was given a special stuffed teddy bear. Finally, Shiloh was bequeathed Oscar's shares of ELQ, but everyone agreed that something wasn't right because Oscar had made arrangements for his shares to fund a charity organization for children that Oscar had wanted Jason to set up. Shiloh was warned that the will would be thrown into probate because Oscar's loved ones were united in their desire to keep the lucrative shares out of the diabolical cult leaders hands.