I have found that 80% to 90% of students do not know that the English letter “O” has more than one sound! The letter “O” regularly uses three different sounds, but a lot of students pronounce many words wrong because they use just one “O” sound all the time.
The basic sounds are: Long-O, Short-o, and Short-o-2.
Long-O
This is the “universal” or “normal” sound for “O” – the sound with rounded lips (most languages use this sound). Some common words with this sound are: go / home / show / short / know / open / low / over / no / most / for / only.
Short-o
This is the normal Short-o sound, and it is actually the same sound as Short-a-2 (as in “mama”). Some common words that have Short-o are: not / gone / coffee / copy / hot / wrong / lot / long / off / on / stop / song.
To pronounce Short-o clearly, the lips should NOT be rounded and the mouth should be open with the tongue low and relaxed.
Short-o-2 (Alternate-Short-o)
Short-o-2 can be thought of as the Alternate-Short-o, and it borrows the sound of Short-u. There are quite a few words with this sound. A few examples are: son / won / from / done / come / some / love / above / nothing / tongue / of / oven / brother / money / month.
To pronounce this sound clearly, the lips should NOT be rounded, the tongue should be very relaxed in the middle of the mouth, and the mouth is less open than for regular Short-o. (See Boss or bus?)
Schwa
Besides the basic sounds, any vowel letter can use the schwa sound in unstressed syllables. However, Short-o-2 and the schwa sound are actually the same (schwa and Short-u are made in the same place in the mouth). So even though we could say the the letter “O” has four sounds, in reality there are only three distinct sounds that you need to make.
Some examples of words with an “O” in an unstressed syllable are: second / complete / contain / observe / produce / melody.
Some tricky cases
- There is a handful of frequently used words that do not have one of the three basic “O” sounds: do / to / two / shoe / who / whom / whose / lose / move / prove. These all use a Long-U sound.
- The words “one” and “once” are unusual. The “O” in these words uses Short-o-2, but there is also a “W” sound at the beginning, so “one” is actually a homonym with the word “won”.
- There are three other words that are very unusual. The “O” in “woman” and “wolf” uses the Short-oo sound, and the “O” in “women” is pronounced with a Short-i sound! The good news is that spellings that are this crazy are rare.
- Also, remember to watch out for “OU” (see OU – Oh no!) – words with this vowel pair are not very predictable.
- Finally, there is one word that can cause confusion because it is a homograph. D-O-V-E: this could be a verb or a noun. As a verb, it is the past tense of “dive” and is pronounced with a Long-O: “dove”. As a noun, it is pronounced with Short-o-2: “dove”.
You should memorize the correct pronunciation of these unusual words, so that you can say them with confidence.
So remember, the letter “O” has more than one sound, and it is usually pronounced with one of the three basic sounds: Long-O, Short-o, and Short-o-2 (or schwa).
ADDITIONAL NOTES
1. Vowel pair “OU” — Several different vowel sounds are used for the vowel pair “OU”, and it is not easy to predict. See: OU – Oh no!
2. Test yourself! — Many students have difficulty distinguishing between Short-o and Short-u. You can test yourself on the “Vowel Test” page of the PronunciationCoach site.
PronunciationCoach 
Why is there a difference in the sound of cost.. Most.. Ghost.. Post
This is the crazy (and difficult) part about English spelling patterns! It would be really nice if all the words that look the same had the same vowel sound. These words use Long-O: ghost / host / most / post. BUT “cost” and “lost” use Short-o.
Here are a couple of strategies to help.
STRATEGY 1. Be sure to memorize the pronunciation of the most frequently used words. The most frequent words have a much higher percentage of unusual spelling patterns. In the words above, “most”, is a very frequently used word, and “post”, “cost”, and “lost” are also fairly frequent. You want to be sure that you are confident about how to pronounce the words that you need to use most often in English.
STRATEGY 2. Use the “probable options” approach to pronouncing new words, which is how native speakers of English handle attempting to say new words. This means that when you see a new word with the letter “O”, you should know that the most likely options are Long-O, Short-o-1, and Short-o-2. There are very few words in which the letter “O” uses a sound that is not one of those three sounds (especially words that are not among the most frequent). So, try saying a new word with the sound that you think is most likely to be the right choice, but be ready to switch to the other options if you find that your first choice is not right.
Is letter ‘a’ in the word ‘all’ said with a short ‘o’ sound?
Yes! Short-a2 sound Short-o (Short-o1) really share the same sound.
Short-a2: water, father, ball, car…
Short-o: not, otter, bother, top…
Some dialects distinguish between these sounds, but even then, they are really very similar and hard for learners to distinguish, so the most straightforward approach is to think of them as the same.
For many native speakers, they are the same, and anyone who learns to speak English this way will be perfectly clear and understandable!