Question

Why do people say kk instead of oki or Mk or just OK?

and why do people say omg instead of Oh My God?

Answer 1

I don't know about the "kk" (probably just to sound cute) but OMG is just faster to type than "oh my god"

Step-by-step explanation:

Answer 2

People use abbreviations for convenience. It takes a lot longer to type out “Oh My God” so people in casual settings just write omg or OMG. Kk and Oki are said just because they’re different ways of saying OK/ok. Sort of like Hello and it’s variations (Heya, Hi, Ello etc...)