Anime First: The manga series that came about only covered a small amount of the anime and changed several things.
Celebrity Voice Actor: In the Mexican Spanish dub, Doremi is voiced by the Mexican soap opera actress Vanessa Acosta. This was a pretty unusual casting move for a Mexican actor, since Mexican Spanish dubs for anime series never uses famous actors compared with other dubs from other kind of stuff (like Western Animation, Disney movies, etc.)
Fan Nickname: Season one is sometimes called Ojamajo Doremi Carnival to distinguish it from the rest of the series. This nickname may have been born out of confusion, since the name of the season's opening theme is "Ojamajo Carnival."
Another fan nickname for the first season is Ojamajo Doremi Mujirushi. The Japanese word mujirushi literally means unlabeled.
Amongst the show's fansub community, the first season is usually referred to simply as S1 (short for Season 1 if you couldn't guess.) Sharp is also frequently referred to as #, since that's how it's spelled in the title. Oddly enough, Motto and Dokkan don't get any nicknames (Unless you count Motto and Dokkan as being short for Mo~tto! and Dokk~an!).
One of the fansubbers here: M and D make for very poor and vague nicknames. S1 and # are much more obvious and stand out as to what they represent.
In the Festival Episode of Dokkan, Hana-chan meets an unnamed boy whom a good portion of the fandom calls Kyo.
That's because his name was given as "Kyosuke" in the sketches of of the Memorial Book. So technically it's kind of canon.
He Also Did: Yoshihiko Umakoshi, the series character designer, also did the character designs for Casshern Sins, a show that is largely different in terms of storytelling and genre with Ojamajo Doremi. He would later go on to work on two more Toei Animation shows, Heartcatch Pretty Cure (which seems to take several cues from Doremi, especially when it comes to stylistic choices and inspirations for certain characters) and Saint Seiya Omega.
That's a typical Magical Girl formula, though, and "Nozomi" isn't an unusual name in Japan...
One of Us: Kento Senga and Toshiya Miyata from the J PopBoy Band Kis-My-Ft2 mentioned on their radio show that they've watched this show growing up, and Senga had to keep it a secret from his friends since it wasn't popular among boys. Miyata has even admitted to having "Ojamajo Carnival!!" as his cell phone ringtone in middle school and being a fan of Onpu.
Talking to Himself: Nami Miyahara voices both Momoko and Masaru, and there is an episode where they interact.
What Could Have Been: Doremi was originally designed with double ponytails before ending up with her double chignon hairstyle. Hana went through several revisions, including outfits that sported a dress instead of shorts, and different hairstyles.
Chiemi Chiba, Tomoko Akiya, Naomi Nagasawa, and Megumi Takamura originally auditioned for the role of Pop. Along with Sawa Ishige, Nagasawa and Takamura also auditioned for Doremi as well.