Here are the lyrics. I believe it's by Andrew Lloyd Weber (and Tim Rice?).

ARTIST: Lionel Bart
Oom-Pah-Pah

NANCY:
All right! All right! 'Ere we go then!

There's a little ditty They're singin' in the city,
Especially when they've been On the gin or the beer.
If you've got the patience,Your own imaginations,
I'll tell you just exactly what you want to hear:

ALL:
Oom-pah-pah! Oom-pah-pah! That's how it goes,
Oom-pah-pah! Oom-pah-pah! Ev'ryone knows:
NANCY:
They all suppose what they want to suppose
When they hear oom-pah-pah!

Mister Percy Snodgrass Would often have the odd glass,
But never when he thought anybody could see.
Secretly he'd buy it, And drink it on the quiet,
And dream he was an Earl Wiv' a girl on each knee!

ALL:
Oom-pah-pah! Oom-pah-pah! That's how it goes.
Oom-pah-pah! Oom-pah-pah! Ev'ryone knows:

NANCY:
What is the cause of his red shiny nose?
Could it be oom-pah-pah?

Pretty little Sally Goes walkin' down the alley,
Displays a pretty ankle to all of the men.
They could see her garters, But not for free and gratis --
An inch or two, and then She knows when to say when!

ALL:
Oom-pah-pah! Oom-pah-pah! That's how it goes.
Oom-pah-pah! Oom-pah-pah! Ev'ryone knows:
NANCY:
Whether it's hidden, or whether it shows,
It's the same, oom-pah-pah!

She was from the country, But now she's up a gum-tree --
She let a feller feed 'er, and lead 'er along.
What's the use o' cryin'? She's made a bed to lie in.
She's glad to bring a coin in And join in this song!

ALL:
Oom-pah-pah! Oom-pah-pah! That's how it goes!
Oom-pah-pah! Oom-pah-pah! Ev'ryone knows:

NANCY: She is no longer the same blushin' rose
Ever since oom-pah-pah!

There's a little ditty They're singing in the city,
Especially when they've been On the gin or the beer.
If you've got the patience, Your own imaginations, I'll tell you just exactly what you want to hear:

ALL:
Oom-pah-pah! Oom-pah-pah! That's how it goes,
Oom-pah-pah! Oom-pah-pah! Ev'ryone knows.
They all suppose what they want to suppose
When they hear oom-pah-pah!