Second time I am asking but I want to be sure I understood the first time. Pronounced omm not oomm?
Not sure about whoever told you that but speaking of proper Arabic, there are only 3 vowels: A I U (and U here is more rounded than O, as in the English word book).
Some dialects in Arabic might say "Im" or simply mama, and even "may" and "immay" i think.
But proper MSA/Classical Arabic is "Umm" (we put double-M here just to make the sound stronger a bit but it doesn't matter really).