Opal Peterson is a Oscar's new girlfriend, the sister of Claudio and Chauncey, the daughter of Dorothy and her deceased father and the niece of Uncle Figaro. She first appeared in Oscar's New Girlfriend and she is voiced by Jenny Slate.
Opal came to Freshwater High to meet the students and greets Milo and Bea. At Lunchtime, she tried to make Oscar happy by going out to Candy Hut afterschool, and buying snow globes at Fish Mall. After Angela spotted him hanging out with her, she arrived at her suburbia home, eating donuts from Donut Duck and she was greeted by Chauncey who was practicing his accordion lessons for the Musical tournament and so as Claudio with his didgeridoo. Her brother opened the door which made Oscar very nervous, Opal convinces Oscar to meet her family, after meeting her family, Opal and Oscar are watching a comedy movie about owls called The Three Owl Stooges which made Opal hold hands with Oscar, he looked at her, he smiles a little. After the movie is over, Opal goes to her bedroom, upstairs and she gave him a wink and leaves.
She is a royal gamma fish, with the purple bodysuit, yellow flippers, fins and curly blond hair, she had an electric blue eye shadow, a green headband with a hamburger on it. Like Shellsea she had a purse, but red with a dark blue flower in the center.
Opal's personality is eccentric, tomboyish, fun loving and energetic teenage girl similar to Francine, Bea's cousin. She is skilled at knitting, painting, cooking, playing rugby, and science, Opal might be more friendly to Bea than Angela as Angela will despise her for impersonating her from Oscar is a Playa. She loves Australian culture, playing with her boomerang, donuts, animals, traveling and spending time with her family on weekends.
- Her dad died when she was eleven years old.
- Her aunt moved to Fish China, when she got a job and talent, selling pottery at the store.
- She and her family adopted Rosie at the pound when she seven on her birthday.
- Opal doesn't have any video games at her suburbia home, only a ball of yarn to make sweaters, mittens and scarfs for her friends, including Angela her friend at kindergarten.
- Opal is best friends with Angela when she was at kindergarten. But later reunited with her when she was watching her brothers doing a music tournament.
- She is a Royal gamma like her uncle and her brothers.
- Opal is called by Mr. Mussels, the Australian Suburban Teenager.
- She moved to Freshwater from Fish Ohio, because her Scottish mother, Dorothy got a job at the library to white chapter books called the Magic Dragons, cookbooks and children's book called Skippy and Zippy.
- Opal had friends that she met from kindergarten joining at their different high schools, Abby Wilson, Wendy Jones, Frank Black, PJ Winifred and Rubella Lopez.
- Her favorite baby animals are ducklings, because she thinks they're cute and puts their name for her phone's password.
Here is Opal's life in the future, according to the Family Path.
- She'll be able to go to a university in Fish Minnesota and learn about designing winter clothes.
- She'll be able to live in an apartment.
- Despite her being a tomboy in Freshwater High, she hates girly things like makeovers, forced to wear dresses, adult contemporary and pop music, pageants and princessss. But she is a secret fan of Glitter Ninja Ponys, a parody of My Little Pony and Teenage Mutant Ninja Turtles franchises.