Douglas Harper's Etymology Dictionary
1877, imitative of bass brass instruments.
Wiktionary
n. 1 (context music English) A genre of Germanic music (especially Bavarian music) typically involving brass instruments. 2 (context music English) A bass line characteristic of such music, a bass that alternates lower and higher notes or chords on the beat, or that alternates in a mid-high-low-high sequence.
Wikipedia
Oom-pah, Oompah or Umpapa is the rhythmical sound of a deep brass instrument in a band, a form of background ostinato.
The oom-pah sound is usually made by the tuba alternating between the root (tonic) of the chord and the 5th (dominant) — this sound is said to be the oom. The pah is played on the off-beats by higher-pitched instruments such as the clarinet, accordion or trombone. Oompah is often associated with Volkstümliche Musik, a form of popular German music, and with polka. In triple time genres such as the waltz it is oom-pah-pah.
The musical Oliver! contains a song named " Oom-Pah-Pah", which is named after the oom-pah.
A more modern variation is the playing of contemporary pop and rock songs in an Oompah style, by bands such as Global Kryner (Austria) and Oompah Brass (UK) (who dubbed the style "Oompop").
Usage examples of "oom-pah".
He turned to walk over to the German Strasse, and was greeted by the oom-pah music of a marching band - why didn't they play the Horst Wessel Lied?