'Ojamajo' is actually a made-up Japanese word consisting of the words "ojama" (a bother) and "majo" (a witch). If you put them together, you get Ojamajo (A Bothersome Witch).
The Ojamajos are represented by different instruments (think "Peter and the Wolf"). Doremi is represented by piano, Hazuki by her violin, Aiko by harmonica, Onpu by flute, Momoko by acoustic guitar and Pop by muted trumpet.