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 use famous actors compared with other dubs from other kinds of stuff (like Western Animation, Disney movies, etc.)
The Danza: Igarashi has the same surname as the director of the show, Takuya Igarashi, whether or not it was intentional.
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.
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.
Singer/songwriter Bree Sharp worked as a writer for the English dub.
The Italian dub has a lot of voice actors who later voiced characters in My Little Pony: Friendship Is Magic: Doremi is Sunset Shimmer, Onpu is Twilight Sparkle, Aiko is Twist, Pop is Sweetie Belle, the teachers are Zecora and Apple Bloom, Momoko is Rainbow Dash...
Keep Circulating the Tapes: 4Kids has not released anything past the first series. In addition, the majority of the original Japanese DVD sets are out of print, making it hard to access the show.
There was once a Portuguese dub in Canal Panda. If you didn't see it while it was being shown on TV, you have lost it forever. Even finding the show from people who did record it is nigh-impossible on the internet. Only some random episodes can be found from this dub.
That's a typical Magical Girl formula, though, and "Nozomi" isn't an unusual name in Japan...
No Export for You: Only the first series made it to the US, and it doesn't seem like the other series, films, and other parts of the franchise will make it (and on the off-chance they do, it's an almost certain fact that 4Kids won't be handling them since Toei cut off all ties with them due to One Piece).
Non-Singing Voice: The Italian dub does it in Season 2; creepily so, having four little girls singing with the same, adult voice. Averted in later seasons, and it's better like that since Emanuela Pacotto, Onpu's Italian VA, is an amazing singer (she also did Utau Hoshina and Twilight Sparkle, always singing in her voice).
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.
To elaborate: In the first episode, Doremi envisioned herself as a Witch Frog; during an early episode of Dokkan, Hana-chan did the same thing. Doremi's actions caused Dodo to fly away in tears during S1; in Sharp, Doremi's actions caused her to leave again, albeit this time in a huff.
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.