Osner's mother, Kristin McGinn, told ABC News on Friday that his right arm had never fully developed because of amniotic band syndrome. The disability led to stares and negative comments from classmates about his appearance when he was in fifth grade.
After realizing the impact bullying had on Osner's mental health and grades, McGinn pulled him out of the Fairfield City School District and enrolled him in an online school during the middle of the academic year.
She said she knew she had to intervene when she asked Osner what he wanted for Christmas and his response was that he wanted friends.
"The bullying started to have an impact on his self-worth and self-confidence, so at Christmas time, I made the switch for him and enrolled him at Ohio Virtual Academy and had him start going to public school online at home," McGinn said. "He became a straight-A student again because he was able to focus on his schoolwork and not the whispers and stares."
Osner told ABC News on Friday that the bullying had taken a huge toll on him.
"It made me want to stay in the shadows rather than being out and interacting with people," he said. "I was just going day by day. I didn't want to do anything special -- just wanted to get the day over with."
In 2015, he started taking taekwondo lessons and noticed how much he enjoyed it. After a year, he started competing nationally and credits the sport for helping him get into a better state of mind.
"This sport has given me the confidence to push my limits and go out of my comfort zone," he said. "I've learned that I can be so much more than I ever thought I could be."
McGinn said that she'd also seen a positive shift in her son's behavior after he started taekwondo. Since starting the sport, he has become comfortable with talking in front of crowds, including speaking at assemblies at his old school about bullying and acceptance, according to McGinn.
"He started off kind of shy and timid and unsure of himself," McGinn said. "In the last three years, he has become way more confident and way more self-assured."
So far, Osner has won medals at state championships in Ohio, Illinois and Michigan and competed against able-bodied athletes in all of them, according to McGinn. He currently trains at multiple locations in Ohio, including West Chester and Dayton, as well as out-of-state locations like Grand Haven, Michigan and Orlando, Florida.
Over the next four years, he will also start competing internationally in hopes of qualifying for the 2024 Paralympic Games, McGinn said.
With October being National Bullying Prevention Month, Osner's message for others experiencing bullying is to focus on those who bring positivity to their lives and the activities they love, in order to limit the impact of the negative comments.
"Don't let it [the bullying] get to you," he said. "Get some friends you can rely on -- you don't have to do this alone. When I was getting bullied, I also had friends that were there for me, and that helped me out a lot because I had people I could talk to."
Meanwhile, McGinn's advice for parents of children who are experiencing bullying is to talk to their children and stay connected with the people who are a part of their children's lives. She said that some of Osner's friends would go home and tell their parents that he was having a difficult time at school and the parents would then call her and alert her about it.
"Talk to your kids -- stay involved," she said. "Kids don't really want to open up about their feelings or admit they're having a difficult time. It can be difficult, but sometimes you have to have those difficult conversations, and don't be afraid to seek help."
Osner hopes his story of overcoming bullying and finding happiness through taekwondo can inspire others facing challenges to find their own passions and have confidence in themselves.
"Find what makes you happy. Find what you enjoy," he said. "Just embrace yourself and life. ... Use all that negative energy thrown at you to do some good as well."