Olivia is generally an innocent, sweet, rambunctious, very brave little girl. She adores everyone, especially her father (who is her only living relative, due to her mother's death). She is fond of fuzzy animals, particularly Toby. She becomes irritated when people say her last name wrong (as is evident when Basil gets it wrong on a multitude of occasions) so she typically corrects them (on one occasion, with the help of Dr. Dawson), although by the end of the movie, it is shown to be an inside joke among them.
While celebrating her birthday, during which she received a dancing mouse doll, Olivia's father was kidnapped by a peg-leg bat named Fidget. She went to look for Basil but did not know where Baker Street was and got lost on the way. She soon met a kind mouse named Dr. David Q. Dawson, who had just returned home after recent military service in Afghanistan when he stumbled upon Olivia. Feeling sorry for the girl, Dawson took her to the home of Basil of Baker Street, where Olivia soon met the housekeeper, Mrs. Judson, who welcomed her in and treated her with great love and care, acting as somewhat of a mother towards her (which was, no doubt, a new experience for Olivia, since she doesn't have a mother). Soon enough, Basil came back to his home, where he was, at first, uninterested in Olivia's case. However, upon her mentioning of her father having been taken by a bat, Basil saw the case as a chance to finally catch his arch-nemesis, Professor Ratigan. After telling Basil the story of the kidnapping in exact detail, it was obvious to Basil that Ratigan had something big planned. As Basil began to ponder what Ratigan would need a toymaker for, Fidget appeared outside Basil's window, a sight that made Olivia scream in terror. As Basil and Dr. Dawson rushed outside to investigate, Mrs. Judson comforted the frightened Olivia. The duo soon returned with plans to hunt down the kidnapper with help from Basil's friend, Toby. Olivia wanted to come with Basil and Dr. Dawson, but Basil wouldn't allow it, saying "this is certainly no place for children." Olivia, however, was allowed to come along (for reasons unknown). When Olivia found out that Toby was a dog, she was, at first, scared, but quickly became friends with the pooch after giving him one of Mrs. Judson's cheese crumpets. The group soon journeyed to a local toy store, where Fidget (who had left his hat when he fled from Basil's home) was collecting toy soldier uniforms for Ratigan's sinister plot. During the trip through the toy store, Olivia was ambushed by Fidget, who quickly spirited her away to Ratigan's lair. She then remains captured for a decent portion of the movie up to the point when Basil and Dr. Dawson come to free her. They succeed in doing so after they escape Ratigan's death machine. Olivia, Basil and Dr. Dawson (after leaving a picture for Ratigan to remember them by) then rushed on to Buckingham Palace, where they saved the Queen from being eaten by Ratigan's pet cat Felicia, and then Mr. Flaversham from Ratigan's goons. Unfortunately, Fidget broke free of his bonds and Olivia soon found herself kidnapped again. She was thrown into Ratigan's blimp, which was soon pursued by a makeshift balloon that Basil, Dr. Dawson, and Mr. Flaversham made out of some balloons and the Union Jack. The chase ended when Ratigan's blimp crashed into the Big Ben clock tower, where Ratigan threw Olivia into the gears of Big Ben, hoping to rid himself of her. Basil hurriedly returned Olivia to her father, but Basil wouldn't escape so quickly. A battle waged on the hands of Big Ben sent Ratigan plummeting to his doom. Olivia believed that Basil, too, had perished (Ratigan having tried to drop Basil several stories to his doom) until he was revealed to have survived by using a piece of the destroyed dirigible. Olivia, having been reunited with her father, left after saying her goodbyes to Basil and Dr. Dawson (which occurred near the end of the movie).
During a Disney Adventures comic that was set in the same year of 1897, but somewhat after the movie, she does mysteries, too, with a reformed Fidget. She states that every time she visits Basil, she knows that something is up. When she went inside, she heard about the Sideshow Sea Beast from Basil and Dawson. She keeps an open mind just like Dawson and called on Fidget and Toby to head on out to the carnival. On the way to see the beast, Fidget showed her how some tricks were done, including how the levitation works.