Timmy Turner was initially voiced by Mary Kay Bergman in the Oh Yeah! Cartoons shorts, with Tara Strong taking the role in season one and onward (and redubbing Bergman's lines in subsequent airings of the Oh Yeah shorts) after Bergman took her own life.
Vicky's sister Tootie was originally voiced by Amber Hood for her debut in the Oh Yeah short "The Fairy Flu", but was voiced by Grey DeLisle for the rest of her speaking roles.
Robert Cait replaced Norm Macdonald as the voice of Norm the Genie for the Fairy Idol special due to scheduling issues.
Butch Hartman replaced Gilbert Gottfried as Dr. Bender (it's fairly obvious, as it sounds like Butch attempting a Gottfried impression, but not being able to get the voice right) in appearances after "Shiny Teeth". Hartman also replaced Faith Abrahams as Francis for his last speaking appearance in the episode "Manic Mom-Day".
Jason Marsden replaced Frankie Muniz as Chester halfway through season 3. Scheduling conflicts could be the possible reason, as Muniz was still busy on Malcolm in the Middle at the time (though, considering Muniz's voice wasn't sounding as youthful as it once was, it could also be because of puberty).
Gary Leroy Gray replaced Ibrahim Haneef Muhammed as AJ after the first two seasons.
Daran Norris originally voiced the Crimson Chin in the Oh Yeah short "Super Humor" before filling in for the Chin's standard voice actor Jay Leno in "Fairy Fairy Quite Contrary" and the video game tie-ins The Fairly Odd Parents Breakin Da Rules and The Fairly Odd Parents Shadow Showdown. He also replaced Brian Dunkleman as the voiced of The April Fool after "Christmas Everyday".
Jeff Bennett replaced Adam West as Catman by the ninth season. This had the added effect of Catman being Adam West In-Universe become something they started dancing around, only referring to him as "some crazy actor".
Remy Buxaplenty's father was initially voiced by Daran Norris, but was instead voiced by Jim J. Bullock for his final speaking role in the episode "Country Clubbed".
Santa Claus has had several recasts throughout the series. He was originally voiced by Michael Patrick Bell in the Oh Yeah! Cartoons short "The Temp", by Kevin Michael Richardson in "Christmas Everyday" and "Merry Wishmas", his de-jollified self in "Christmas Everyday" by Tom Arnold, is voiced by Jim Ward for his one line in "Boys in the Band", by Butch Hartman in Fairy Idol and by Carlos Alazraqui in "Dad Overboard".
International dubs:
In the Latin American dub, with the exceptions of Timmy, Crocker, Anti-Wanda, Foop, Norm, Nana Boom Boom, Blonda, the Easter Bunny and Sparky, every character has had at least two voices. However, Vicky, Mrs. Turner, Elmer, Sanjay and Binky recovered their original voices after some episodes.
Despite the complicated history of the Hungarian dub(s), Erika Náray remained as Wanda (and by extension, Anti-Wanda) throughout, though she spoke with a much different tone in the original Nickelodeon dub. Timmy and his dad on the other hand had as many as three voice actors.
Timmy's German voice actor, Hannes Maurer, moved out of Germany in 2015. He was replaced by Christian Zeiger for Season 10.
Brazilian Portuguese: Timmy was first voiced by Alexandre Drummond, who was replaced by Thiago Farias after he hit puberty. When Farias retired from voice acting, he was replaced by Luiz Sérgio Vieira.
The French dub moved from Belgium to Paris after 2005, because Nickelodeon had acquired the rights to future seasons from Nelvana. The Parisian French dub started in 2007. While Nessym Guetat (the Belgian voice of Cosmo) returned to the series in season 10, he instead voiced Mark Chang and Chet Ubetcha, while Cosmo himself is now voiced by Fabrice Josso.
Pascal Renwick, the first Parisian French voice of Doug Dimmadome, died in 2006. He was replaced by Georges Caudron.
The Italian dub has side characters pass between different voice actors many times. Chester's first voice actor actually returned after some seasons, while Jorgen Von Strangle and Vicky's changes lasted one season each.
When The Fairly OddParents! crossed over with The Adventures of Jimmy Neutron, Boy Genius, this caused a major problem in Latin America because both series are dubbed in different countries (OddParents in Miami, Florida; and Neutron in Mexico). Nickelodeon reached a compromise by dubbing the first crossover in Mexico with the cast of Jimmy Neutron and its two sequels in Miami with the OddParents actors.
The Italian movie of the three crossovers is an interesting thing: the Jimmy Neutron cast already had two different voice casts (one from the movie and one from the series), so what happened? Since both the movie and FOP were dubbed in Rome while the series was dubbed in Milan, one would imagine that they just used the voices from the film dub... except it's a bit more complicated. Most of the Jimmy Neutron characters had the voices from the series dub, except for Jimmy's parents and Sheen who have their voices from the movie dub, and Carl has a completely new voice actor since the one from the series dub died.
