Oscar is the main protagonist of Shark Tale.
In his youth, Oscar greatly admired his father Earl's work as the number tongue-scrubber at the Sykes' Whale Wash so much, he decided to bring it to school for show and tell, only for the class to sarcastically satirize him. This tragic disaster caused him to consider himself a loser and a "nobody". Since then, Oscar dreamed to be rich and famous as a respected "somebody" by enriching himself with enough Clams to buy a penthouse at the top of the Southside Reef, where all the somebodies lived.
Now an adult, Oscar had just recently succeeded his father as top tongue-scrubber at the wash. However, his constant get-rich-quick schemes over time got him into tough situations involving a series of debts that cost him over five thousand clams from his boss Mr. Sykes, yet Oscar didn't appear to be bothered by it. Arriving late for work one day, he found that he was already punched in on the time clock, and realized he was been helped out by his best friend Angie, who now worked as the Wash's receptionist and cashier (harboring a secret unconditional crush on him). He stopped by her office to thank her and chat, exasperating her with his latest get-rich-quick-scheme: bottled water. Angie advised Oscar to just do his job "which, by some miracle, you still have." Before leaving though, Oscar gave her the breakfast he brought her.
As the day progressed, he bemoaned his existence shoveling slime in the gaping maw of an endless line of whales. Even worse, a prank on his fellow tongue-scrubbers turned unpleasant when a whale was accidentally hit in the eye with a jet of soap and thrashed about in pain. Oscar closed the emergency clamps to hold the whale immobile and squeegeed the soap from the his eye, shortly before Sykes' jellyfish henchmen Ernie and Bernie arrived and hurled him into their boss's office. In order to raise the cash as a means of paying his great white shark crimelord client Don Lino protection, Sykes demanded Oscar to pay the five thousand back. A shocked Oscar asked how he can possibly come up with that much money with so little time, but Sykes dismissed his worries and told him to bring it to the Reef City Seahorse Racetrack tomorrow, or else. Oscar questioned "or else what?" to which Sykes responded "The boys'll explain." As soon as the pufferfish left, Bernie stung Oscar in the eye as a warning. That evening, Oscar moped on Angie's balcony as she tended to his swollen eye. Because she couldn't understand why he was always getting himself into these situations, nor his obsession with trying to live at the top of the reef, Oscar recalled the mocking he received from his classmates of his admiration of his late father's tongue-scrubbing work. Despite implying that he wished to become a somebody to prove them all wrong, Angie tried to explain to him that he didn't need to live at the top of the reef to be a somebody, and, feeling sorry for him, tried to help him out by giving him a pink pearl – a precious gift from her grandmother. Oscar was reluctant to take such a valuable object from her, but Angie insisted, saying it would get him the money he needs to pay back Sykes.
The next day, Oscar pawned Angie's pearl and arrived at racetrack with the five thousand clams, but became distracted in his dreams of fame. Upon hearing that the race was rigged, he foolishly bet all the clams for clams one of the famed seahorse at the race, Lucky Day, to win instead of paying it directly to Sykes. Such a million dollar bet was noticed nearby by a beautiful superficial lionfish named Lola, who flagrantly seduced him an exited state. However, upon finding out that he was a whale washer by the time Sykes turned up with Ernie and Bernie, she exposed the bet to everyone and left, leaving Oscar disappointed. Sykes was furious that his top tongue-scrubber placed bet on the money he was owed, but nonetheless agreed to see how the race turns out. Sure enough, it went bad moments before their betted horse could cross the finish line since Lucky Day instead tripped and fell onto the line, thus letting a rival famed seahorse named Fish Fingers win the race instead and forcing Oscar to see his dreams of begin rich and famous evaporate. On Sykes' orders, Ernie and Bernie took the young fish to a wasteland miles from home, tying him up to a rock and torturing him with their glowing stingers. At the same time, a vicious young great white shark named Frankie was approaching nearby trying to teach his vegetarian younger brother Lenny how to become a fish-killing shark. When he spotted the jellyfish toying with Oscar, he told Lenny it didn't come any easier than this, and instructed him to eat the tied-up fish. Ernie and Bernie fled as Lenny approached and psyched himself up to eat Oscar, but couldn't go through with it. He tried to fake it by biting through Oscar's ropes and thrashing his tail to stir up a cloud of silt to hide Oscar's escape. But the little fish was confused by Lenny's attempt to save him, and was still there, looking baffled as the silt settled. Furious, Frankie tried to attack Oscar himself when suddenly, an anchor fell on top of him, crushing him to death. Lenny swam off, distraught with grief and guilt, leaving Oscar with Frankie's body as Ernie and Bernie returned. With no other witnesses were present and Oscar seen near the body, everyone came to believe that he killed the shark, an opportunity that Oscar realized that he could exploit for his wanted fame, leaving them with this belief.
The tongue-scrubber returned to the city with the new nickname "the Sharkslayer". Sykes became his manager, Lola became his girlfriend, and Oscar himself moved to the "top of the reef" to live in luxury. When several sharks got close to his neighborhood, Oscar's neighbors expected him to drive them away. After scaring the sharks off, he on the way back ran into Lenny, who forced him to let him stay with him when the shark became aware of Oscar's lie. Thus, Oscar let him sleep in a warehouse next to the Whale Wash, where Lenny explained his troubles about being so different and harmless from the other sharks, which led to Frankie getting killed. The fish told him that it wouldn't have happened if he was out there in the first place. Lenny found that sweet, but said that if his and Frankie's father learned about that, he would ice Oscar for sure. This confused Oscar, who asked if Lenny's dad was like a mafia boss or something, to which Lenny replied "Yeah, he is". This struck extreme terror in Oscar as this response brought him to realize the identity of Lenny and Frankie's father: Don Lino! Scared to death, he returned to his penthouse to find Ernie and Bernie playing a Sharkslayer video game (and losing each round) and Sykes talking on the phone with Lino, telling the mob boss that he had become untouchable in his role as the manager of Oscar, who the pufferfish added will slay him (Lino) and all of his great white sharks if they didn't start paying him (Sykes) protection. Because he was furious of Frankie's supposed murder, Lino asked to speak with Oscar and tell him that he already sent his sharks after the fish to "tear you fin from fin". The threat Lino made was briefly interrupted by his octopus second-in-command Luca, who butt-dialed him thinking that he was calling to order from a pizza joint until being told otherwise due to his boss ordering him to get off the phone. Sykes was happy now that Oscar's new fame saved his business from getting wiped out by Lino himself, but Oscar tried to warn him of getting it all wrong now that he fears Lino's wrath himself. Lola, having just popped up from another room, massaged him of his worries and advising him not to think of going back to the bottom of the reef, saying that he should show the sharks whose boss, and they might leave the reef alone.
Letting the her fibs going to his head, Oscar returned to the Whale Wash warehouse to check on Lenny, only to find him drinking soda and an unpleased Angie confronting them both, having just demanded Lenny into telling her everything that got Oscar to bring him into the wash. When she tried heading off to expose the lie to everyone, Oscar and Lenny convinced her to keep quiet, though she was heartbroken by Oscar's dishonesty. Explaining the other situation involving Lino, Oscar and Lenny staged an event in which Lenny pretended to terrorize the town and Oscar defeated him throwing him into the depths of the ocean. By the time the sharks (led by Luca) arrived and saw what happened, Oscar scared them off and everyone cheered for him as his fame was cemented even further, even when Lola swam up and forcefully kissed him in front of the cameras, causing Angie to leave in anger, jealousy and sadness. Back at the warehouse that night, Angie furiously argued with Oscar over is lies and fame resulting in her finally confessing that she'd always loved him, "before the money, and before the fame". She then tells him that she no longer saw him as a somebody but now a "fake, sham, con and a joke". Oscar, deeply hurt, tried to say something, but Angie told him to go away as she was "tired of hearing about how everything you had in your life wasn't good enough; including me". He sadly swam out of the warehouse, while Angie wept. While swimming around the reef, he saw how much his lie affected him far more than he intended it to and what a jerk he had been. He saw the Shorties kids and and his old hermit crab neighbor Crazy Joe laughing and remembered how fun the bottom of the reef was like and how he still enjoyed it. He loooked at his penthouse and his smile quickly faded as he sadly entered it, ignoring the party inside and shutting the balcony door behind him. When Lola came out on the balcony, Oscar rethought his feelings and dumped Lola, which unfortunately led to her great fury as she mercilessly whacked him against the windows before sulking away.
The next day, Oscar bought some Valentine's Day gifts for Angie and was about to present them to her, only to be informed by Sykes (who he found organizing the stuff in her office at the Whale Wash) that she had gone missing. The phone rang and the caller, who turned out to be Luca, hinted othat Don Lino kidnapped her to force a sit-down. Lenny came along now disguised as a dolphin named Sebastian. They arrive at the meeting to find Lola (who obviously arranged the whole thing) next to Don Lino, while Angie is tied up and gagged and presented to Don Lino on a plate who prepares to eat her if Oscar doesn't comply. In response Oscar has Lenny (disguised as a dolphin) to grab Angie and eat her on the spot (grabbing her in her mouth, but not swallowing), but Lenny begins choking and vomits Angie out. Lenny's disgust in fish makes Lino recognize his youngest son. Lenny in defeat, sheds his dolphin disguise. Lino, however is embarrassed of the situation and accuses Lenny of "taking sides against the family". When Oscar tries to defend Lenny, Lino's sadness quickly turns to fury and he begins to chase Oscar, snapping his jaws at him.
During the final battle, Oscar leads Lino towards the Whale Wash. He pulls down a lever to trap him into a whale clasping device, only to discover that he trapped Lenny by mistake. Lino continues to chase Oscar. Eventually, Oscar subdues Lino by trapping him into another machinery, leaving him face to face with Lenny.
Oscar is rather kind and affectionate to his friends, but due to his ambitions, he can make wrong decisions and even come to hurting his friends (although its unintentionally), he later realizes his mistakes and tries to make things right.
He has an extreme low self-esteem since as long as he can remember as he was made fun from his classmates because his father was a tongue whale scrubber.
Oscar is also somewhat of a liar as he had everyone convinced he had killed Frankie, earning him the title of the Sharkslayer.