There are multiple sounds using the letter O. In most cases, you make this sound with the rear portion of the mouth. You start with your mouth in an open state. Your tongue should also be away from the top part of the mouth. Then you make the sound by bringing in the lips in a circular manner and the tongue moving upward and towards the front of the mouth ending right in the middle.
If you combine the "oh" and "oo" sound, you have the sound of the letter "O". Along with these sounds, it is common to use "O" with other vowels such as "OU" as in Hour. We will see examples for all these types below.