Or Nah is a slang term that became popular after the song "Or Nah" by Ty Dolla $ign ft. The Weeknd, Wiz Khalifa & DJ Mustard came out. Or Nah is means "Or No" in plain Amerian English. Soon after the song came out, young kids, mostly those under the age of 18 started saying "Or Nah" after asking a question.
Or Nah Example
After someone ask their friends if they're going out, that individual might say "Or Nah" right after the question. When someone say "Or Nah" they usually put a lot of emphasis on the Nah so it comes out sounding like "Naaah".
Majority of people consider "Or Nah" to be an annoying fad just like YOLO. This term is a trend, young kids love joining trends, but just like any trend, it will die out.