I know. This sound is not existent in Arabic, nor Persian, nor Standard American English.
It's very difficult to explain it this way, because speaking/ listening require actual physical face-to-face contact. But anyway the following may help:
-British /O: /
The most important feature of this sound is its strong lip-rounding. Produce a sound between /o/ and /u:/. autumn = /O: t m/
-American /a: /
This is the one that you're familiar with. However, the /a:/ is usually accompanied with a very weak W-like sound. That's why some dictionaries represent it by /aw/. The /w/ in the American allophone is used to show some slight lip-rounding. autumn = /a: t m/, /aw t m/