The Oh Yeah! Cartoons shorts and first season were far different in compared to what came after:
The shorts were animated using cel animation compared to the main show which was animated digitally.
Da Rules book was colored yellow instead of purple in the shorts.
Vicky had more deeply saturated red hair compared to the orange ginger hair she would have later.
Timmy was far more of a sarcasticStraight Man compared to his more optimistic personality seen in the final show. He was also voiced by Mary Kay Bergman in the original shorts. When the series was picked up, Timmy was voiced by Tara Strong due to Bergman's death. However, when the shorts were re-aired on television until the original airing of "The Big Super Hero Wish!", Strong's voice was re-recorded over Timmy's lines.
In "The Fairly OddParents!" pilot Timmy had red hair the same shade as Vicky's, in every other episode Timmy's hair is brown. Word of God is that this was changed to prevent Timmy and Vicky from being mistaken as siblings.
Cosmo's and Wanda's current personalities were reversed in the shorts: Cosmo being the more down to earth one and Wanda being the ditzy spouse, though neither really came across as dumb (contrast crazy and stupid). Cosmo also had a more smooth baritone Phil Hartman impersonation, which even the start of the official series had distanced from for an increasingly shrill, helium pitched voice (Word of God said he was supposed to sound like a used car salesman).
In the first season alone, Cosmo and Wanda were then turned into a pair of cheerfulwacky fairies ("two halves of a whole idiot"). With "Transparents!" showing Wanda, being nearly as much of a CloudCuckoolander as Cosmo and does just as mediocre of a job as Cosmo at trying to act like a human.
Vicky acted more like a bossy normal teenage girl, with the worst she did was give Timmy chores to do and take away his fun (like forbidding him to watch TV until his chores were done) and Blackmail him. A far cry from her more well-known charactization of being a crazed psychopath who tries to kill Timmy.
Vicky mentioned having a little brother in the pilot. However, he was subsequently replaced with a little sister, Tootie. Speaking of Tootie...
Vicky's sister Tootie had a noticeably different design during her debut in the Oh Yeah! Cartoons short "The Fairy Flu". Her head was more oval-shaped, her pigtails were a different shape, she wore pink glasses frames as well as a blue dress and she didn't have braces.
Vicky and Tootie's relationship was also different compared to the final show; Vicky actually stands up for Tootie, something Vicky in the final show would outright never do, and she even brings Tootie to Timmy's house while Timmy is recovering from his allergy attack.
Timmy's parents always have their faces hidden offscreen, to the point where their original model sheets depict them as headless. Their outfits are also considerably different: Mr. Turner wears a teal three-piece suit and brown shoes, while Mrs. Turner wears a red dress and high heels, gold bracelets, and a pearl necklace.
In one of the pilot shorts, Batman Can Breathe in Space was played straight. The show proper would avert this.
In the shorts, Jorgen was portrayed as a regular Fairy Godparent (with a very scared godson named Winston) instead of being an authority figure of Fairy World.
The Tooth Fairy was depicted with red hair in her first appearance rather than blue.
Cupid's appearance in the Oh Yeah! short "The Zappys" depicts him as blonde, gigantic, overweight and wearing little besides a diaper.note though this design does appear on and off again in the series proper, notably the episode "The Same Game" In the series proper, he looks more like a standard fairy, has pink hair and wears a pink coat in addition to a diaper.
The Crimson Chin's first appearance is in the final Oh Yeah! short "Super Humor", where he notably lacks the black spot underneath the C insignia on his chin that he'd be depicted with in the actual series.
A few title cards in the Oh Yeah! shorts ("Too Many Timmys" and "The Temp!") were animated, and one had no title card at all ("The Fairy Flu"), just the title on a moving image. The show proper used static title cards for all its non-special episodes (except "Super Bike" and "Old Man and the C-")
In both the Oh Yeah! shorts and all Season 1 episodes, the ending cards have a saying somewhere that says "Made in Hollywood U.S.A!". This was dropped starting with "Christmas Everyday!", as at that point, post-production departments switched to CCI Digital.
Cosmo and Wanda need to use human electricity to power their wands for an especially complicated wish. In any other episode after that one, they draw their power from the tower in Fairy World.
If Timmy, Chester or AJ lose all their lives in the video game world, they would die for good. Later episodes would establish that fairy magic can't be used to kill/seriously harm someone.
In "Spaced Out!", Cosmo and Wanda "borrowed" Mark Chang after Timmy wished for an alien for his friends to play with, claiming that they can't create an alien out of nothing. In later episodes, creating entire eco-systems out of nothing is almost not worth mentioning, since it happens so often.
In "A Wish Too Far", Trixie briefly mentions having a mother. Later episodes imply that her father is a single parent (he shows up often with no wife in sight).
In "Father Time", Timmy uses the heat vision he wished for earlier to win a race. The second season would introduce the "can't use magic to win a competition" rule, though it could have been added after the events of that "Father Time".
Season 2 onward
In early seasons, Timmy losing his fairies didn't include having his memories erased ("A Wish Too Far", "The Switch Glitch" and "Abra-Catastrophe!". It wasn't until the Season 3 episode "The Secret Origin of Denzel Crocker!" that this became an aspect of losing one's fairies.
In "A Mile in my Shoes" judging by the angry look on Dinkleberg's face, Mr. Turner's hatred for him was mutual.
Speaking of Dinkleberg, in "Fairy Friends and Neighbors" Mr. Turner actually invited Dinkleberg over, and it was the latter who wasn't interested this time around.
When the Anti-Fairies made their debut in "That Old Black Magic", Anti-Cosmo had normal-looking teeth. All subsequent appearances would depict him with fangs.
Dr. Rip Studwell was depicted with Eyes Always Shut in his debut episode "Hard Copy". Subsequent appearances depict him with his eyes clearly visible like everyone else.
