Hey there danglo,
The nice thing about oom pah is that it's a concept of accompaniment that can stretch to fit with countless variations. The basic oom pah idea has three components.
The first is about pitch. Oom pah means low high, low high etc. The low note will likely be the 1 of the chord triad (though 3 and 5 are possible). The pah could be the octave above (my first choice), the fifth above (my second choice), the third above (my third choice) the same note as the oom (my fourth choice), another note that sounds good (my fifth choice). Perhaps I might use two or three notes that sound good for the pah, but certainly not always, usually it's just one note for the pah as well as the oom.
The second component is about button duration (how long you hold your finger down on the button) and that can be expressed by this simple formula: long short, long short etc.
In addition, it's good to note that when possible, the melody notes should have an even longer duration than the accompaniment notes. Right hand long, left hand short is the rule for Anglos. For simplicity, think about three button durations. Melody is the longest, oom is in the middle and pah is the shortest in duration. This concept is hard to master on the box, but key to clear playing that lets the melody come forward singing.
The third component is about accent. In cut time (most tunes, like reels and hornpipes are in this) you make the oom be accented less that the pah. That should sound like this: "oom PAH, oom PAH" etc. I'm talking about a measure of time which can also be counted "1 and, 2 and" etc. When I say "accented" I mean an extra umph on the bellows. All four beats of a measure counted "1 and 2 and" should have a little extra umph, but the "and" beats should be stronger than the ones and twos. All of this piece of advice can be expressed in a simple sentence... "accent the off beats to get lift and drive in a dance tune".
There you have it. To actually do it, practice each bit separately to isolate your task.
If you are able to play with all three of these things going on at once, then you will sound really cool and that special individual of the gender you prefer will fall in love with you, there will be many babies and you will die a happy grandfather. Ignore my advice at your peril.
(Then there all the other cool things you can do that are not oom pah... like playing in octaves, thirds and sixths... harmonies high and low and all quite possible on the 20 button.)
Edited by Jody Kruskal, 16 March 2016 - 02:11 AM.