• Olivia (voiced by Emily Gray) is a young pig and the main character of the show. She is imaginative and fantasizes about different roles, such as a pop star or superhero. She displays on setting up good examples and shows kids how to share, use their imaginations, be physically active, and be self-confident. She is in 1st grade.
• Ian (voiced by Michael Van Citters) is Olivia's younger brother. He looks up to Olivia and enjoys being included in her activities, but often becomes the typical annoying "little bother". He likes, among other things, dinosaurs, robots and baseball. In Season 2, he becomes less annoying, is more intelligent, on better terms with his sister, has a small planet on his T-shirt and a deeper voice.
• William (voiced by Robert Toonitititusa) is Olivia and Ian's youngest brother and typically sleeps, eats and cries.
• Mom, Olivia's mother (voiced by Joyce Beverley (Season 1) and Jennifer Reiter (Season 2)), who runs her own party-planning business from home.
• Dad, Olivia's father (voiced by Danny Katiana) is an architect and occasionally absent-minded. He often provides his paternal wisdom to Olivia and her brothers in "little talks".
• Grandma, Olivia's maternal grandmother (voiced by Yvonne Craig). In her late-fifties, but still fun and adventurous.
• Uncle Garrett (voiced by Connor Hall), Olivia's maternal uncle, is a professional football player and is a bit of a comedian. He performs ballet with Olivia and only appears in "Olivia Takes Ballet".
• Grandpa Cedric, Olivia's unseen grandfather that Olivia mentions in "Olivia's Day at the Office".
• Perry and Edwin, Olivia's dog and cat. Perry is energetic and loves to play with the children, while Edwin is generally lethargic and prefers to sleep.
• Goldfish, a goldfish whose owner is Ian. It is rarely seen.
• Cedric, a one-time frog that Ian found and named after his Grandpa.
• Julian (voiced by Jeremy Herzig) is Olivia's male best friend. He's smart with a self-deprecating sense of humor, but shy and lets Olivia take the lead. While often reluctant to go along with Olivia's ideas, he finally always does.
• Francine (voiced by Brianna McCracken (Season 1) and Cameron Escalante (Season 2)) is Olivia's female best friend. In the first season, she can be snobby, villainous, tries to make Olivia feel jealous at times, and often serves as the antagonist. She acts more "girly" than Olivia and will not touch anything gross. Her birthday is eight months after Olivia's. In Season 2, Francine gets better and friendly to Olivia and her classmates.
• Gwendolyn is Francine's yellowish orange tabby. She is very talented and likes Edwin. In one episode, Francine reveals that she sometimes dresses her in doll clothes to have a tea party and makes Gwendolyn fish sticks.
• Herman is Francine's dog. He makes his only appearance in "Olivia Trains Her Cat". He can ride a skateboard and also knows tricks like Gwendolyn.

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The shorts are small, generally wordless segments in each episode (two per). They include the following:

William Eats (And Burps)
• William is hungry so Mum puts him in his highchair and goes to get him some applesauce. While she is gone, everyone else gives him tidbits of food. At the end, Mum says she thought he was hungry, but it appears William does not want his applesauce. She is surprised when he burps.
Olivia in the Bathroom
• While Dad is shaving, Olivia keeps taking her stool, getting up on it and doing something in front of the sink and mirror such as brushing her ears. At the end of the short she gets on her stool and kisses Dad.
Olivia and Her Toy
• Olivia tries to get her favourite doll to stand up straight, but it won't so she tapes it to the wall and the two have a tea party.
Olivia Scares Ian
• Olivia tries to scare her little 'bother' but she can't time it right. Later Ian scares her.
Olivia Builds a Tower
• Olivia is building a tower of random items. Ian, Perry, and William try to knock it down, but Mum and Dad forbid it. At the end, it's Olivia who purposely knocks it down.
Olivia Cheers Up William
• There are two versions of this short. In one (used during the first season), William is upset and Olivia tries to cheer him up. At the end (when Olivia is in her cow costume), she hurts her leg by stepping on a block and cheers him up. The other (used during the second) is similar, except with Olivia laying Edwin beside William instead of hurting her foot.
Olivia's Bookbag
• Olivia is stuffing her bookbag for school with Perry and Ian taking her things then returning them each time she leaves the room. In the end, her bookbag becomes too heavy for her to carry and when she puts it on she falls over backwards.
William Goes for a Ride
• Olivia puts on her train hat takes her family on a ride in her wagon. First it's just William, who is then joined by Perry, then Edwin, and finally Ian. The wagon gets heavier as each kid and pet climbs on board. At the end all the kids and pets are pulled in the wagon by Dad.
Ian Wants to Play
• Olivia and Julian are jumping rope and Ian wants to play, also. Throughout the entire short Ian is trying to get their attention but he is unsuccessful. Finally at the end he gets their attention and Olivia and Ian jump rope.
Olivia's Hidden Talent
• When Olivia sees a bowl of oranges she discovers she has a hidden talent for juggling. When she throws four oranges in the air they disappear. When she looks up, she realizes Ian who is standing on the stairs above her has caught them.
Olivia Slides
• Olivia and her toys slide many different ways in the park.
Perry Plays Fetch

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In addition to airing on Milkshake! and Nick Jr. in the UK and TG4 In Ireland, the series was aired on the Nick Jr. Channel from 1 February 2009 until 30 October 2015 when Nickelodeon and Paramount Global lost the rights to air the series. The show came back on air and was broadcast on Universal Kids in the United States, Disney Junior in Latin America and Brazil, and Treehouse TV in Canada.