오 is a Close-mid back protruded vowel(similar to Vietnamese ô sound) -Its vowel height is close-mid, also known as high-mid, which means the tongue is positioned halfway between a close vowel (a high vowel) and a mid vowel. -Its vowel backness is back, which means the tongue is positioned back in the mouth without creating a constriction that would be classified as a consonant. -Its roundedness is protruded, which means that the corners of the lips are drawn together, and the inner surfaces exposed. 우 is Close back protruded vowel(similar to Vietnamese u sound) -Its vowel height is close, also known as high, which means the tongue is positioned close to the roof of the mouth without creating a constriction that would be classified as a consonant. -Its vowel backness is back, which means the tongue is positioned back in the mouth without creating a constriction that would be classified as a consonant. -Its roundedness is protruded, which means that the corners of the lips are drawn together, and the inner surfaces exposed. So, the difference of 오 and 우 is the position of the tongue in the mouth. If the tongue position is high, it sounds like 우. If the tongue position is mid, it sounds like 오.