Some words that contain the "short O" sound are mod, cot, block, top and box. Vowels in English have both long and short forms. In contrast, words with the "long O" sound include vote, mope, joke and woke.
When teaching phonics to children, teachers often use "long" and "short" vowels to help children understand the different sounds that each written vowel represents in written English. The first lesson a student usually learns about the letter "o" is the sound it makes when it is short: /awe/. Short "o" usually occurs when the "o" comes by itself in the word, meaning that there is no "silent e" at the end of the word.
The next lesson a student learns about the letter "o" is that its long form sounds the way the letter is said: /oh/. You can usually tell a vowel is long when it is near the end of the word and precedes a consonant and an "e." This is true of all five vowels.
There are a number of pairings where a silent "e" at the end of a word changes both the sound of the vowel and the meaning of the word, such as "lobe" vs. "lob," "mope" vs. "mop," "tome" vs. "tom," "mod" vs. "mode" and "wok" vs. "woke."