* Formal: Omahepo, Mama! (This translates directly to "Hello, Mother!" and is the most respectful way to greet your mother.)
* Informal: Mwa! (This is a casual way to greet your mother, similar to "Hey Mom!" in English.)
You could also use Mwa! followed by Mama for a slightly less formal greeting.
Ultimately, the best way to say "Hi Mom" in Oshiwambo depends on your relationship with your mother and the context of the greeting.