Why do people say kk instead of oki or Mk or just OK? and why do people say omg instead of Oh My God?

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...)

Yes because in graphic design you add elements of art into one piece and/or program. although, you place together graphic designs on a computer,  you still have to know about the principles of design which is included in art.