So then it's Ootsutsuki. Here is how my mind went about it...
The 't' in the name is silent.
Ootsutsuki => Oosusuki.
Over the years, the second 'su' became 'ma' ,due to some shift in heritage. Therefore,
Oosusuki => Oosumaki.
As decades passed and by hearsay, 'Oo' became 'U' and 'su'...