I find that the /o/ sound is a kind of universal for human to describe things that are big or sublime. (something above our level)
Say, in Japanese big is 大きい which reads o-ki-i with an /o/ in English and many others there are "awe" and "awe"-some Now in Dutch, I know that there is o-ma.
This word in quite beautiful and I suppose it to be a quite primitive word with an anthropological origin.
Last but not least ... the sentence its self is a very sad one... for the mother :)
This is actually a sentencr I got to use once. Short story time! Once I went out with a friend and in the evening, when he had to go home, my mother offered to take him home. And when he left him home, he texted me to thank me and added "your grandma looks like such a nice person!" My reaction was "I- uhm. She's not my grandma. She's my mother."