From the video description:

The division of OM into A-U-M originated in the ancient Mandukya Upanishad where the three letters represent three states of experience – waking, dream, and sleep. But rules of Sanskrit grammar require the letters A and U to combine into the letter O.

According to the rules of sandhi (euphonic combination), the letter a followed by u must combine to form o. They can never remain side by side. For this reason, to say that om has three letters, aum, is incorrect. The Mandukya Upanishad's represented OM as A-U-M for symbolic purposes, not for pronunciation or writing.

Sanskrit vowels are generally short (hasra) or long (dirgha). But the o of om is prolated, that means it's extra long, it is three units (pluta) in duration. So, om is properly pronounced with an extra long o, om.