Hi, may I ask whether there is any rule on when o is pronounced as /o/ (there should be a dot above "o", according to Merriam Webster's Dictionary), and when it is pronounced as /a/ (there should be two dots above "a", according to Merriam Webster's Dictionary)? You can check  / for details.

For example:

The "a" sound: font, almond, honor

The "o" sound: autumn, long, caught
I know what you call the "o" sound is often supposed to be spelled "aw" or "au", so most words with "aw" or "au" should be pronounced with that sound, in theory.

In practice, things are very different. There is a feature in American English called cot-caught merger, which means those two are pronounced the same. In particular, there is no distinction between that "o" and that "a", and they both come out as one vowel, so there isn't a distinction anymore. This merger is pretty common. I adopt it in my English, so I don't make a distinction: in general, I use a vowel close to what you call "a" for everything, dog, cot, caught, long, hot, etc.

If you want to maintain the distinction, I guess you'll have to try to be consistent and remember which word has which vowel. I don't think there is a fixed rule, there are just too many regional variations to consider.
JingtianFor example:

The "a" sound: font, almond, honor

The "o" sound: autumn, long, caught
You are asking how an "o" is pronounced, but your examples include, "a" and "au". ( ? )
I would characterize the sounds differently as follows:

The lax "o" sound: font, etc.
The "au" sound: long, etc.

And there are other pronunciations of "o" in addition to these two, for example:

The lax "u" sound: from, son, front, above, etc.
The tense "oo" sound: move, tomb, do, etc.
The tense "o" sound: go, most, comb, etc.

Note that the "au" sound is not used by all Americans. They substitute the lax "o" sound. So you don't really need to learn it unless you want to speak like Americans in the eastern part of the country. In the eastern U.S., syllables that end as shown below are pronounced with the "au" sound, and other combinations take the lax "o" sound.

-off, -oth, -oss, -ost, -ong, -og

So in that way of speaking, the "au" sound comes up in these: scoff, coffee, moth, boss, loss, cost, lost, long, song, dog, log. And the lax "o" sound comes up in these: lop, sob, cot, rod, mock, don, mom, lodge, botch, doll.

But it's all right if you use the lax "o" on all of those.

Careful: When "o" comes between "w" and "r" the "or" is pronounced "er" as in "her":
word, worth, work, worm, worse.

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There is probably no consistent rule, but the following is worth remembering.

Written "al" has the "au" sound before "k", "l", and "m".

al + k --

walk, chalk, talk, stalk (as if wauk, chauk, tauk, stauk)

al + l --

wall, fall, call, mall (as if waul, faul, caul, maul)

al + m --

calm, palm, balm (as if caum, paum, baum)
But some people pronounce the L in this combination -- as if caulm, paulm, baulm.

All of these are also commonly pronounced with a lax "o" instead of the "au" sound.

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Thank you, Kooyeen and CalifJim. Your replies really help me a lot.
One more follow-up question: Sometimes the letter a has the "a" sound, but sometimes it has the "o" sound. Is there any rule for this?

The lax "o" sound:  almond
The "au" sound: almost
 CalifJim's reply was promoted to an answer.
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Thank you.

I think I need to take a dictionary with me wherever I go to check the pronunciation.Emotion: smile